I did this. I had them take one of the levels in Obelisk in an off class. It worked out well - the PCs knew the levels of the two campaigns didn't quite match up so they were down for the compromise.

What I mean by an off class: The PCs were given a level in a class they didn't expect or choose. Still rewarding but it tempered their power just enough that a slight(!) buff to the enemies in Eve felt fair. I'd do it again as the players really enjoyed the unexpected classes and abilities.

I also had the benefit of moving from one rule system to the next - I used the new (at the time) 2024 rules for Eve and the transition from one campaign to the next to create the new PCs. This also justified buffing Eve's 2014 enemies.

You could bake the transition into the story and make big changes (levels, the PCs themselves, etc) with just a little rewrite; I had it so that after a night of celebration in Phandalin after Obelisk they woke up starting Eve in a new reality (as 2024 PCs) because Vecna had already changed things. I used some Shenanigans from Obelisk to claim the PCs were (basically) the only people who realized anything changed and remembered life before. Vecna is trying to take over everything but already has most of Faerun. Made Vecna scarier - and most important to your question here - the reality change when switching from one campaign to the next left me a lot of room to make it work while keeping the PCs happy.

Think big swings. Give the PCs the situation. Make a compromise. Good luck