New Campaigns & Levels that work in [legacy] only.

Campaigns & Levels [Current Version]

Uninstalling Mods does lose progress in [legacy], but, if you've done a Backup and if they're for [Current Version] only, that shouldn't be an issue.

Going back to [legacy] will make your Saves erase themselves, so make sure you backup and keep seperate Saves for both Versions so you don't lose anything.

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A thing to note, if you've installed Mods for [Current Version], they cause an Error:
"Steam Workshop content update failed. Please restart the application to try again."

This can prevent you from installing Mods that would otherwise work as, when the Game enumerates its Workshop Content in [legacy], it does so in the order you've subscribed to them and stops enumerating when it encounters a single Error.

So, keep being subscribed to the [legacy] Mods, it doesn't matter even in [Current Version], but to have new Content for [legacy], you need to unsubscribe from the [Current Version] Mods and any other ones that would cause an issue, such as "Additional Laser Colors", you can tell as it tries to enumerate a problem Mod for a second and then shows an Error. After enumerating the Mods you want, it's OK to resubscribe to the [Current Version] Mods, maybe, because even if they'll cause an Error Message, well it's not like you would've been able to play them.

There also shouldn't be an Issue if you went back to [Current Version], then to [legacy], it doesn't seem to have to enumerate all the Mods again. To make sure though, look at how many Mods it fails to enumerate; right now I have 6 Mods exclusive for [Current Version], so if [legacy] enumerates 1/6 Mods before failing, it's correct and I'm good to go.