Unfortunately it isn't as simple as just swapping out the batteries for a new solution. If it's currently running 4 AAA batteries then you're running the LEDs off of 6v and they are properly resistored for that amount. If you use 2 AAA Li-Ion (and yes they exist, just a different name) then you're talking baout 7.4v. A significant difference for sure. If you're looking to change battery solutions your best bet is to find someone with electronics know-how (maybe you already knew all of this so you might be able to fix it yourself) or contact Deep at
[email protected] and ask if he can upgrade your battery pack. In the end some serious changes will have to be made.
Cheers.