Games like FIFA (outside that 3 months period) or GTA won't charge you as a DLC the next gen upgrade, they'll charge you the full game even if you bought it for XBO.
I think most games not published (or under a marketing deal) by Microsoft (or Sony) won't give you the next gen version for free. Same goes both for Sony and MS.
If CD Project Red will be upgrading Cyberpunk 2077 for next gen for free, same for Ubisoft with AC Valhalla, anyone can do it.
Yes, everyone will be allowed to do it in both PS5 and Series X. But I think that it will be like with Crossbuy between previous PS home consoles or portables: I think only a small percent of the games will support it because to make games is very expensive and they are already making an effort selling each game AAA for $60.
This kind of shit has to go honestly, this is the worst thing that happened this generation, the need to have a spreadsheet with all the multiple game versions.
It just says that the next gen version will be released later because the new consoles won't be in the market on September, and that the base game will be $10 more expensive on next gen. And well, that they'll have a $99 special edition. Nothing too complex, bad or new.