Nixon can change his mind, no one is stopping him. He just forfeits 1 year of NCAA Division I eligibility. Decisions have consequences. All Division I programs have agreed to this stipulation, not just Iowa. I am certain if he asks, Iowa will release him from his LOI. They don't want a player if he doesn't want to be a Hawkeye.
But what about Bahma and their tampering. This is a serious infraction and should result in the tampering a University or coach being penalized. And, shouldn't the offer from the University that gets caught tampering be disallowed?
Sadly, this means Nixon could not get a scholarship (or allowed to play football) for the first year he attended Alabama if he chose to, with no guarantee that Bahama would have one for him the next year, or he would have 4 years of eligibility to play for any other school who offers.
Decisions, decisions, decisions.