I certainly managed on a relatively new install of home premium to upgrade to pro just by entering the product key, it then did a few updates and has been fine ever since.
If it's an old install then it might be carrying a lot of baggage so a clean install would likely be better.
As yours sounds like a new install I'd go with the key change.
I don't think I actually used 'anytime upgrade', if you already have a new valid key, just enter it via 'control panel', 'system', near the bottom of the screen is 'windows activation' with a link to enter a new product key.
Call me 'w23'
At any given moment in the universe many things happen. Coincidence is a matter of how close these events are in space, time and relationship.
Opinions expressed in forum posts are those of the poster, others may have different views.
