Hello, this is Emily.
You will need to use your Microsoft account to to claim the free upgrade. So yes, it can be linked to your Microsoft account after that. But you can always go to Settings > Accounts > Your info, and click on "sign in with a local user instead" to revert back to using local account.
Just keep in mind though, some features that requires Microsoft account sign in in this case won't function properly, such as Sync settings, etc. But if you've been using Windows 10 with a local user all this time, you already know that.