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?