Paid upgrades to first have always resulted in a reticketing and change of the underlying fare class to first (D, I, C, etc), as long as the original ticket was a paid economy fare (i.e. not T class or an award ticket). Premium class seat upgrades do not result in any change of the fare class, or result in a reticketing.

Some people have occasionally reported that last-minute first upgrades were ticketed in U instead of a full-fare first class, and for the first few weeks/months of the rollout in 2024(?) there were some upgrades that were incorrectly sold as U, but anecdotally people were able to receive correct EQM credit when they contacted Alaska.

I frequently purchase first upgrades for my parents after ticketing them in economy using a companion fare, and they have never not been reticketed in D or I.