I have a PSE Mach 10 with 70# limbs which are too heavy for me.  I found a set of Mach 10 60# limbs but I need to know if there's anything else I need to know before buying them? If they are both Mach 10 limb sets they should be the same size and swap no problem right?  I just want to make sure there aren't different length limbs or something that could create issues.

If nobody chimes in with direct knowledge, the best bet would be to call PSE and ask.

It should be a direct swap, but who knows.  Also, there is a chance that a 50-60# bow uses a different cam than its 60-70# brother.

Aside from that, you'll need a method to safely relax the limbs completely and destring the bow so you can disassemble the bow.  Typically, you need a real bow press to do this.  Although, it's possible with a Bowmaster portable press if you get creative.

They sold the limbs while I was searching for an answer so no need anymore. I'm just going to put it back in the case and store it way or strip the accessories off and toss the bow since it has no resale value.