Feasibility of swapping two MacBook Pros (both hard drive and RAM)? I have an early 2011 MacBook Pro, and my sister has a mid-2012. I've already seen the suggested duplicate possibilities that say that just swapping the hard drives shouldn't be problematic, but I've got another layer: I've got 10GB of RAM, an eBay-bought 8GB module and one of the OEM 2GB modules, that I'd very much like to keep.

Wikipedia says the two generations have different RAM specs - the early 2011 has "1333 MHz PC3-10600 DDR3 SDRAM, Voltage 1.5V" and the mid-2012 has "1600 MHz PC3-12800 DDR3 SDRAM, Voltage 1.35V." I'm going to take an educated guess that mixing frequencies/voltages won't end well, but what if I swapped the pairs of RAM modules? As in, what would happen if I were to put 10GB of RAM designed for early 2011 into mid-2012 and 4GB of RAM designed for mid-2012 into early 2011?
The RAM speed won't be a problem, the system will detect the speed of the RAM and run it at the maximum compatible speed. Hard drives also won't be a problem. (you will want to do a "fresh" install of the operating system)

As for the RAM voltage, I think that would be a problem. I'm not completely sure though. Whatever you do, make sure you run the Apple Hardware Test and babysit the machine while its running. Chances are if it passes all the tests, you're good.