[user104658]

Originally Posted by James

Oh, I see... I think.

What happens if Cap changes a historical fact in the past then? (after returning the stones).

Well one theory is that he would just be careful and live a quiet life with Peggy, and she would keep it secret.

Second I guess is that The Ancient One implied that the Infinity Stones actively keep the timeline intact so it would be impossible to drastically change any "key moments", as a "fate" of sorts keeps everything more or less on track. They also confirm that predestiny / fate exists in the MCU as The Ancient One already knows who Stephen Strange is (and that he'll be the Sorcerer Supreme), long before he meets her.