nic.wise:
I've done an SSD replacement in a slighly-older iMac - wasn't easy, but wasn't too hard either, with the kit to do it (mostly to remove the screen). I can loan you the gear if I can find it :) Made the machine quite usable, and a 512GB SSD isn't overly expensive anymore if you need a bit of space - I have a 128 in my Mac Mini, as most of the storage is on the NAS anyway.
Not sure I'm quite feeling that adventurous, but I'll think about it :-)