I'm going to say something horrifically gauche: Why not let them actually PLAY their BB characters in RotRL?
Yes, it will make the first module ridiculously easy. But where do they end up at the end of the BB? 4th level? By midway through the second module they'll be roughly where they're supposed to be, and they'll have had a blast walking over their poor unmodded enemies. Once they start hitting real enemies (the big X at the end of Module 2), they'll be very happy for the extra levels.
Just a thought...
Or you could get the players to shelve the BB characters (complete with XP and loot) until you have the chance to run a non-AP adventure at the right level for them. After all, Paizo has lots of modules which are not part of APs.
Also, there are some now-classic moments during Burnt Offerings which might fall a bit flat if the party is already equipped with certain items from the start.
If the players just want to keep their XP and loot, just have the ones who want to keep their characters do so, and have players who want to make new characters create them at level 2.
If the issue is more that they like their character fluff and backgrounds, let them create new characters with the classes they like, and retcon the history so they're the ones that went through the beginner box adventure.
Here's how I might handle it, if my players just really wanted the XP and loot they scored in the beginner box:
Let those who want to recreate their characters do so, and retcon it so that's how the party always was. Everyone starts at 2nd level. Like people pointed out, a jump to 2nd level isn't incredibly unbalancing, and might increase survival with a small party.
Use a plot device to get rid of most of their loot. You could use the "we were robbed!" trope, or ask your players to come up with one, if they're mature enough to work with you and come up with an idea that involves nerfing their characters. Personally, I would leave them the healing wands and take all the other big items (half plate, dragon slaying sword, acid gem).
I know most players don't like DM fiat that penalizes them, but depending on how mature your players are, I think since you are working with them and giving them an early XP boost, they won't feel too badly about losing their items.
If you want, you could make the robbery into an ongoing plot thread. There are plenty of unsavory characters in Sandpoint who could be culprits, and if you string this out a little bit, you can even give them back their items at a more appropriate level. If you do this, I might give a necessary clue to an NPC who happens to be far away from town and hard to track down (the ranger Shalelu might be a good candidate). That way you can prevent your players from just abandoning the main plot thread and focusing all their energy on getting their stuff back.