I said "Upgrade, capped".
I almost said "Fixed", people can work out, study, etc. and improve themselves in a fundamental way, in real life. It's had to accept absolutely static score because of that.
I really hate the stat bloat that's happened in 3e and later, though. Stat changes should be a rare thing, though. Not every character should have them and never by more than a point or two (cap at +2, total, over the life of the character). You also shouldn't be able to exceed racial limits -- that's why they're called limits. People who start maxed out have already done the perfect workout routine (or whatever) that allowed the other guy to increase his.
Spells to raise stats should not be particularly common or standard issue, but they do make sense. Maybe move them up to 4th level spells giving a +2 and requiring higher slots for higher stats (hmm... meteor swarm or +4 str?).
Wishes and other extreme events are an exception, but they should be just that -- extreme.