Can I swap out an Ultegra 6750 crankset with a 6700?
The spindles should be the same width and diameter. Seeing as how you already have an Ultegra crankset, you should be able to use the larger Ultegra crankset without switching the BB or getting an adapter. But you will have to move the front derailleur up and probably re-cable it. Switching will make you have to constantly move it so you'll have to leave enough at the end when you're in compact so it'll fit when you move to standard.
If you're looking for more gearing but don't feel like switching everything back and forth, you could always switch to mid compact chainrings (52 x 36).  They use the same BCD as compact and it should be once and done without having to move your derailleur every time you switch.