Likely they think that the upgrade is mainly (maybe exclusively) for people playing via game pass. The $5 more is because you didn’t buy the game to start with. They did the same with FH5 when that launched I believe.
I’m with you on pre-ordering but GTA and Elder Scrolls games are my two exceptions. Not sure how I feel about Starfield, should be similar to an Elder Scrolls game since there is no online stuff to go wrong.
My whole hearted advice is to wait for reviews before ever buying a game. Therenis really absolutely zero reason to ever pre order. The pre order bonus skins or whatever are playing on your emotions. People need to resist!