Upgrading a campaign

[Glangath]

I'm currently the dungeon master if my group in princes of the apocalypse, and considering which campaign to lead next. Either curse of strahd or out of the abyss. But my players feel like their characters are meaningless if they have to make new ones after the current campaign. So I'm considering "leveling up" the campaign to fit a party of level 13-ish and playing through 20.
What I'm looking for are suggestions to what to think of, and how to proceed with that process.

[uraniumrooster]

I think it would be easier to increase the levels in Out of the Abyss. The story would also make more sense for higher level characters than it did for 1st level characters...

The party starts out captured in a drow slave pen... and we're supposed to believe a 1st level party can escape?! From a Drow stronghold?! It took some suspension of disbelief, and a lot of fudged die rolls on my end, to let 1st level characters survive the intro to this mod.

[Democratus]

If you have the gumption, writing your own campaign for their remaining levels is the best option.

You know the players and the characters, what they are like and what they wish to accomplish. A custom campaign can cater to all of that while also being level appropriate.

[Falcon X]

There's so much that can kill you in the Underdark. Out of the Abyss should be a natural adaption. Heck, you can even start at the second half of Out of the Abyss, having another adventuring party tell you what they know.
Rise of Tiamat (skip Hoard of the Dragon Queen) is also viable.
I would hesitate to do Strahd because it's hard to pull off horror/survival with vampires once you are at higher levels and easy to mess with the tone. Not undoable though...

If you are willing to go out of 5e to adapt, there are lots of old high level adventures that would be fitting. That's what I did with my Tyranny of Dragons Game (Hoard of the Dragon Queen<Ravenolft<Vecna Lives<Rise of Tiamat<Die Vecna Die. Very long story of how I chained them together).

[Glangath]

Thanks guys! Those are some great ideas!
I'm probably going with OotA since the underdark is such a dangerous place.

[Eldamar]

I think it would be easier to increase the levels in Out of the Abyss. The story would also make more sense for higher level characters than it did for 1st level characters...

The party starts out captured in a drow slave pen... and we're supposed to believe a 1st level party can escape?! From a Drow stronghold?! It took some suspension of disbelief, and a lot of fudged die rolls on my end, to let 1st level characters survive the intro to this mod.

The book gives multiple ways for the party to escape... None of which depend on character level.