I have seen many great assets on the workshop, and in many comment sections I have read people saying "I would have liked this one better as a park instead of a Unique Building" or "Why didn't you make this a cemetery?"

So, help is on the way in form of this guide.

With this guide, you can just edit a workshop item, and for example change it from unique building to a park. (Note: You will not be able to turn a prop model straight into an asset or vice versa)

I will describe here, how I repurposed many of the items on my workshop.

You will need all of these mods:
Asset ItemClass Changer by Snow_Cat
Building AI Changer by Cerebellum
Asset UI Priority Changer by Snow_Cat
Mod Tools by BloodyPenguin/nlight
UI Category changer by SnowCat

If the respective workshop item that you want to repurpose doesn't have an icon (asset_thumb) or tooltip (asset_tooltip), you will also need:
(Asset thumbnail) Sprite Changer by Snow_Cat
Toggle Asset Properties Panel by Snow_Cat
...and an image processing software (photoshop, gimp, paint, etc.).

Before you start, make sure you haven't just subbed to the mods mentioned here, but also marked them active in the content manager.

From the main menu, go to tools, select asset editor and click on "new".
Select a map too (which one doesn't really matter as all props, and therefore all kinds of trees will be loaded).
In the next UI, go to "building", and select the asset that you want to repurpose. Click on "load" and confirm to import the standard model.

In the asset properties UI to the right side of your screen, you can now click the green arrow (no pun intended, it is what it is) and select the new item class. Parks, for example, are listed as "Beautification item", as can be seen in the screenshot.

Click the green arrow next to the UI category and select the new item category.
This decision can seem a little bit tricky, as many UI categories are not known to the common player by their designation, since the UI category includes sub categories.
Important:
When you are unsure which UI category to pick, either choose "default", or even better: Load another asset that already is right in the UI category you want your repurposed asset to be and select the same.

First of all, you need to understand what the UI priority does.
The UI priority is a number that determines the order in which your assets will appear within its respective UI category. So if you want to have certain items to be listed first or last you do that with the UI priority.

The way it works is this: The lowest number will be the first item to be displayed within each UI category. The item with the highest number will be the last item to be displayed within that UI category.

So let's say you made 3 items in the Parks category: A small park (UI priority -1), a medium park (UI priority 0) and a large park (UI priority 1). Now when you are in the game and open the UI to place a park, those 3 parks will be displayed from left to right in the order small, medium, large.

So in order to make it easier for everyone, you should definitely assign a rather unique UI priority, which means: Don't pick numbers with less than 3 digits.

If two assets have the same UI priority (many have 0 or 1), they will switch positions among each other upon every load.

So, it makes sense to have all your assets have a similar UI priority, let's say your first one gets UI priority 346 and every new one within the same UI category gets +1. This way all the items you created will be next to each other (Unless you are unlucky and someone else picked a number inbetween your assets, this is why you shouldn't pick low numbers) and therefore easier to find.

At this point you might have already noticed that despite having adjusted the Item class from "Unique Building" to p.e. "Electricity Wind Turbine", the stats of your asset are still those of a unique building (or whatever kind of asset it was before).

Therefore, you must enter the new AI manually by typing it into the text input field marked in red.

In order to know what to type in here, check out this List of valid AIs by Snow_Cat.

After typing hit enter and the AI will change. You will now be able to adjust the fitting category's stats.

Sometimes, an asset that you want to edit might not have a tooltip or an icon. The more workshop items you gather, the more annoying this gets. Therefore, please make sure your item has both before you release it on the workshop. Also, don't forget to add a snapshot.

In order to assign a new tooltip or icon, this is what you need to know:

• Icon and tooltip are located in the same folder and currently - if at all - can only be accessed via the folder-button in the "save asset" dialogue.
  
• The icons have the name "asset_thumb.png" and are 109x100 pixels. (Mouseover variation files are created automatically.)
  
• The tooltips have the name "asset_tooltip.png" and are 492x147 pixels.
  
• In general, it is possible to use png-file with larger measures, BUT: There is a size-limit for these files, which, if exceeded, will prevent you from uploading your asset to the steam workshop. I don't know the exact limit, therefore I recommend sticking to the measures provided above.
  
• Assets that are missing thumbnail (icon) and tooltip will not display the button to open the secret folder. For assets missing only one of the two, the folder can be opened, but the asset will not save the thumb/tooltip graphic to the new asset and instead keep the original thumb/tooltip, even if one of these doesn't exist.

So after you finished creating your tooltip and thumb pictures with your image processing software, create a new asset (when you create a new park asset, the button for the secret folder will always be displayed, and tooltip/thumb will always be saved) and just save it with the images you want to assign to your other, incomplete asset.
Then click on "Copy" within section marked red in the screenshot. Load your other asset, and click "Paste". Now your new tooltip/thumb will be saved when you save the asset.

Most items have to be placed along a road.
There are however some assets which you might want to place somewhere else. Just somewhere not along a road, or even on water.

For this, we will utilize Mod Tools by BloodyPenguin/nlight.

While the asset is loaded in the asset editor, press F7 to start the Mod Tools UI.

Now choose your placement mode from this list and copy the code into the text input field of the mod tools console (left of where it says "Run").

• On Ground (=Anywhere on ground)
  
  ((BuildingInfo)GameObject.FindObjectOfType<ToolController>().m_editPrefabInfo).m_placementMode = BuildingInfo.PlacementMode.OnGround;
• On Water (=Anywhere on water)
  
  ((BuildingInfo)GameObject.FindObjectOfType<ToolController>().m_editPrefabInfo).m_placementMode = BuildingInfo.PlacementMode.OnWater;
• Roadside (=Anywhere along a road)
  
  ((BuildingInfo)GameObject.FindObjectOfType<ToolController>().m_editPrefabInfo).m_placementMode = BuildingInfo.PlacementMode.Roadside;
• Shoreline (=Needs to have both water & ground)
  
  ((BuildingInfo)GameObject.FindObjectOfType<ToolController>().m_editPrefabInfo).m_placementMode = BuildingInfo.PlacementMode.Shoreline;
• On Surface (=Can be placed on water and on ground, but doesn't need to have both)
  
  ((BuildingInfo)GameObject.FindObjectOfType<ToolController>().m_editPrefabInfo).m_placementMode = BuildingInfo.PlacementMode.OnSurface;

When you copied the code into the input field, hit "Run". Then save your asset.

Please do consider:
I am not involved in the development and/or continuation of these mods.
This guide is from 2015.
And I haven't played the game in years and am rarely online in Steam.
So...
If you do have any questions feel free to ask them here, someone else might be able to help you out, but it's unlikely I can help you.