I'm sure they can come up with whatever technobabble they need to reset the characters if they decide to send them back for good. However, I'm deeply suspicious that the end result will be something to the effect of the characters getting duplicated someone and one copy being sent back in the past to become their regular adult selves and the other copy stays in the present to continue being the temporally displaced O5. Why lock yourself into one path when you can have your cake and eat it too? (Personally, I think the O5 need to go back for good, but I'm nonplussed with most of the classic X-Men characters when they're not in movies or TV shows anyways, so it's not like I was ever really enthused about the idea in the first place.)