Thats the problem. You need to remove the two barbs from the block before you swap out the mounting plate.
For me this will be ok, because the only water left in the loop is in the res (the pump wasn't on long enough to circulate the water around the loop), so I'm not that bothered if I have to remove the barbs from the block.
If on the other hand, the loop is completely full of water, then its a pita. You'll have to drain the loop (you will be thankful for that drain port you installed

) before you do anything. And of course re-fill and bleed the system on completion.
How difficult did you find it bleeding the loop?
Is this the process that you used for filling / bleeding the system (I'm just curious):
Leave the fill port open (in the top of the case)
Completely fill the res.
Turn on the PC (and pump).
Top up the liquid levels as the pump gurgles away.
Tilt the case from side to side and shake it a bit to get all the air out from the nooks and crannys in the rad and CPU block
Keep topping up the levels until the liquid backs up slightly up the fill port tubing (i.e. the res and loop is completely filled)
After a couple of minutes the pump stops gurgling and becomes a lot quieter.
Put the plug in the fill port.
Job done.
It was a piece of cake when the loop was out of the case for the bench test. It'll probably be a bit more difficult when everything is inside the case.
Edit: From the picture it looks as if you can remove the mounting plate, without having to remove the acetal (black) or acrylic (clear) top to the block, which the barbs screw into. Thats unless you want to remove it, to have a look inside
