If you create a new training scheme and delete the Standard you get a message that all the players will be assigned to another scheme but that actually never happens(also recruits come with no training scheme assigned).
If a player is not assigned a training scheme and you press the Save button on the left, the game suddenly closes. I think an error message like "all players need to be assigned to a scheme" would be more fitting than the game crashing 😁.
At the end of the 2022 season my Senior QB requested a trade(despite playing every game, winning every game of the season, and this being his last season in college) and transferred to Alabama. I paid no attention to it knowing that he was going Pro. I got the Pro Draft notification and he went first overall. Next season I was surprised to see he got drafted 1st overall again, but this time with Alabama. Unfortunately I have no screenshot when the trade happened since I thought it was irrelevant.
Disclaimer: I'm European so my knowledge of how college football works is limited, but I think that once you go Pro you can't go back.