I've never been inside an RM50, but I can tell you that the board is extremely easy to remove on an RM100. All I had to do was remove the chassis from the headbox, remove the density and presence knobs, undo the nuts that hold the pots to the chassis, and slip the board out. The board is only about three square inches, and is connected to the rest of the amp with a pair of molex-type connectors. I recommend using pliers to get a grip on the plug and work it out of the socket, rather than pulling it by the wires. I also recommend taking a photo of the board before you disconnect it from the rest of the amp so you can remember which plug is which. You don't have to get near any big capacitors while doing this (but of course, be very careful that you don't somehow accidentally do so anyway).
For me, the most difficult part of doing the mod was extracting the old pots, as I didn't have any desoldering braid, and my desoldering pump wasn't going a great job getting rid of all the solder. I ended up flexing the old pots back and forth until the pins broke off, which allowed me to remove the pins one by one. I then needed to file off a bit of the remaining solder from the inside of the holes in order to get the pins to fit; an old guitar string would work for this. After that, there wasn't much to it (I didn't change any caps). My RM100 is much, much brighter now than it was before.
