Hi folks
Just got the upgrade; wanted to report that none of my workspaces/projects (ex: when you press Ctrl+Alt+P and pick from drop down) are working 
They are all reporting errors like this in the console:
Ignoring invalid workspace /C/Users/brian/.sublime/.workspaces/bb1.sublime-workspace
Ignoring invalid workspace /C/Users/brian/.sublime/.workspaces/bb2.sublime-workspace
Anything I can try?
OK weird - solved.
Somehow, all my project/workspace files got deleted! 
I checked some of them in with git though, so it was easy enough to recover. No idea how this happened. I literally had one project open when doing the upgrade (from 4107) and when upgrade launched my open workspace was trashed. I also lost session data (open tabs, etc)
I had installed in the portable format to C:\Users\brian\.sublime and files were in a folder there called .workspaces. This folder was completely deleted The ones I didn’t have in git are lost
UPDATE: I found the folder .sublime (4107) in the trash can. It looks like everything in this folder (which is where I store workspaces) was just moved to the garbage can unfortunately. I can simply copy them back
It’s never a good idea to store your own files in the installation location of an application because most are going to assume the only thing putting files there is them and will move them with impunity.
If you’re using a portable version of Sublime, you probably want to do something like that inside of the Data folder; that’s where the configuration is stored and would be left alone during an upgrade.
@OdatNurd good advice!
To anyone reading this who may be doing the same: I installed the ProjectManager package which puts them into the user package directory. It also takes care of clearing the quick project menu (which is a PITA to rename after you move projects)