I have just been asked an interesting question, and I could not answer for sure.
Suppose you are running 8.1, then upgrade, and rollback in the free period.
We all know that establishes a digital licence.
We also know we can clean install at any time in the future.
Also, we know we can upgrade aa second time in the free period.
BUT can we upgrade again (keeping all files - forget wisdom of doing this) after the free period ends?
My gut feel is we must be able to do so, as you can already upgrade an unactivated version of windows 8.1 but the upgraded version would remain unactivated assuming no prior digital licence.
I assume the upgrade would check for a digital licence anyway. In worst case, you might need to use tbe Redstone Activation Troubleshooter.