"though he'd cease to be" that's assuming in this manga they're going off the split timeline/multiverse theory. If they're going off one true timeline the change would just be made, he'd be able to go find jae-ha in the present once the event that caused him to die early was no longer relevent.
I think..if the past is like a tiny fragment for Yeon's present time then it probably would not affect his current timeline..But if Yeon's decision to save Jaeha made a big impact then i'm sure the timeline would change. If this is not gonna be complicated then I hope Yeon will stay in the present and find that Jaeha is still alive somewhere.
but obviously different. i hope hes able to change the past...though he'd cease to exist, because then they'd both be happy.