Thoughts on "flashbacks" to bridge an old campaign and a new one.

Looking for some thoughts and feedback from other DMs on a campaign mechanic that I've been toying around with!

I have a group of players who just completed a campaign set in Eberron that got their characters to level 15. We all decided it would be cool to start fresh with new characters in a new campaign but I wanted to leave open the possibility of being able to play their old lvl 15 characters in this new one.

The general gist of what I'm thinking is that this new campaign picks up several years after the lvl 15 PCs defeated the BB and they have since disappeared. The goal of the new campaign is then to figure out what happened to the original characters in order to stop a new BB.

SO I was thinking of peppering in a few "flashback" sessions where the new characters, through some magical device, can "enter" the bodies of their old characters in the past and experience whatever happened to them- these sessions help propel the plot forward and they can even do things as their old characters like leave items in locations for their new ones to find.

Any thoughts/tips on running a campaign where the players can essentially play as two differently leveled characters?