I have a USP compact Da/Sa and I read that slide will fit on a p2000 frame. I would prefer to buy a p2000 with LEM trigger but I dont know what is different on the inside of LEM guns. Are all the differences in the frame? If the slides are any different this obviously won't work and I'll need to buy the p2000 in Da/Sa.
Model to model they interchange as far as I know. Never heard of a USP Compact slide being interchangeable to a p2000 frame, however. I know the magazines should be interchangeable, and the trigger groups have small differences.
I'm not sure exactly what you want your end result to be. But to answer one of your questions, yes, all the differences between da/sa and LEM are in the parts installed on the frame. In case you're not aware, you can buy a kit with all the parts required to convert your USPc from da/sa to LEM.
I would not say ALL the differences between LEM and DA/SA are in the frame, but if we want to be factual, let's not forget the FPBS that lives in the slide. So there you have it.
Oh, this is the first I heard of the USP slide being compatible with a P2000 frame. And even if it is compatible, I would not want to be the one shooting it live. My hand(s) and fingers are more precious than some coins (and they don't grow back, either). Save the money and buy what you need. Don't compromise.