Fantastic information mate. Thanks very much for that. Looks like a lot of work changing all of those LEDs out of 3 units!!! Fair play.
I'm guessing it's not a difficult job lifting the diodes off the thermal paste? I've seen a few people struggling with it.
Did you replace the paste and if so was it the grease/paste you can buy from eBay?
It took some time, but I kinda did it one at a time as I played with the layouts before settling on this. I bought an extra fixture so that I can have one on the bench changing out diodes and not take any off the tank; then just swap them out once its done. Rinse repeat.
As for difficulty, it depends. Two of 4 of my fixtures came with diodes attached with thermal adhesive to the board, so once you de-solder the two legs (+/-) it was still a pain to pull them off. I used locking pliers as well as a jewelers chisel to pop them off...both methods are still a pain and you may pull off some of the board protective layer (doesn't effect function once everything is done). The other two, once the legs were de-soldered, diode was held in place with just thermal compound and came off with minimal effort. So I guess it's the luck of the draw.
To mount them back, yes, add a thin layer of thermal compound between the board and the diode. I use MX-4 just because that's what I have on hand. Then just pop them on and solder the legs. Done. Takes me about an hour to do 1 fixture now.