over 4 years ago - Avanya - Direct link
I believe they will just disappear. Potentially you'll have some errors pop up (one per building that's disappeared) when you load, but you should be able to click them away, and loading up next time shouldn't have them.

If you do it, then definitely save as a new file when you have swapped to boreal, so if something unexpected happens, you can swap back to the winter version. :)