I thought replacing the board wouldn't be difficult, but it was unexpected things that caused issues, mainly with the back plate. The M7 is assembled with the assumption that it wont be disassembled at any time, making parts of the job very frustrating. It's really a job for someone experienced in DIY, including plenty of experience with case work.
The M7 digital board is longer than the R7.1 digital board, so the small shield box around the power socket has to be removed. There is enough slack in the internal power cable (itself in a shielded tunnel underneath the digital board) to take off the back plate and pull it out a couple of inches. However, due to all the glue covering the solder joints, either you have to replace the power socket (preferably with one that uses nice, solid core pins) or you're in for a nasty, fiddly job of removing solder mixed with glue. The internal power cable also has a dielectric that is far from ideal for wire that is soldered, as it melts and stretches, so in the end I put crimp terminals on them, covered with heat shrink so if ever I have to, I can remove the back plate without having to deal with soldering and de-soldering that wire. Later on, while I was in Yodobashi Camera, I found the perfect speaker/power wire which has dual sheaths in between which is a shield. I now wish I'd replaced the internal power wire with that instead.
The rest of the board replacement was fairly straight-forward, but quite fiddly, as you have to do things such as prop the digital board up while you solder the power wires on and the front and rear panels have to be worked on while half-off. I ended up using Scotch tape to hold them in place at an angle while I soldered the wires in.
