An inplace upgrade should not affect files, etc.
True, in place upgrades would (or should) leave existing files... well... in place, but the question has a bit more to it.
I haven't upgraded to 11 yet myself, but I'm on Win 10 Pro, so I know I can upgrade without needing an MS account.
However, my wife's PC is ready to upgrade. It's a Dell AIO that is only 3 months old, so it already has Win 11 downloaded and ready to install now (got the notification about it a couple of days ago). I just haven't done it yet, and she's not ready for it either. However, since that is running Win 10 Home, it's gonna want the MS account if we do decide to upgrade it.
Now, when I first set it up, it was on the recent version of Win 10 that forced me to set it up with her MS account (I had gone too far by the time it was asking for the account for me to go back and find that point where you could by-pass it by skipping the wi-fi). But after I was done, I put it back to the local account like her previous machine had so I could directly copy back her files from her old machine into her local account name under /Users/.
So, that question is, if I do upgrade to Win 11 on that machine, will it add the MS account user name under /Users?
If so, can I still change the system back to her old local account name and maintain the document saving location back to that existing local account name? I would assume that the local account would still be on the system in Win 11 since it would be an "in-place" upgrade.
In the case of my wife's computer, she has a primary (C:/) SSD drive with the OS, and secondary (D:/) HDD where the data files are saved to her /User/
local_account_name/ folder.
I think this is sort of what the OP was asking about as well.