I’ve done a number of these - going from 4 or 6 core up to 12 core. Straightforward thanks to this guide.
Makes the 6,1 a more attractive unit to me as you can buy one inexpensively and then update the storage (2 TB Intel 660p most recently) and the CPU (E5-2697v2 was $180 on eBay), and RAM (if necessary). I’d go for a D500 unit - the D700 and D300 models seem to have the most issues.
Swapped a E5-2667 V2 into a base quad-core 1620. Considerably faster on multicore tasks. ;)
Not difficult at all - take your time and have some sort of organizer for the various screws you remove. And a torque wrench is a good thing to use so you don’t under- or over-tighten anything.