I know the foam well, have had an Antec H2O 920 that went in two different rigs and an NZXT Kraken X60 that has been on three different motherboards now... the most recent one I simply didn't bother using the foam because it is such a pain to remove.
My best method I have found, get a flathead screwdriver that is big enough to not gouge but small enough that it has more of a chisel end to it.
If you are worried about scratching the motherboard get small piece of electrical tape over the end of the screwdriver, if you are concerned it isn't a bad idea, but I just took my time and was careful enough to not worry about it.)
With the motherboard upside down lift one end of the plastic backplate and use the screwdriver to pry/peel the sticky foam off the motherboard. ALWAYS get it off the motherboard first. With care and patience the foam will come off, just gently lift the backplate and pry the foam from the motherboard side and lift tiny sections at a time... eventually you will have one side off!
Once you have one side if jam something under that side so that it doesn't stick down again while you do the other side. TRUST ME! It will stick back down and you WILL be annoyed. Preferably get something that isn't conductive or likely to scratch the motherboard.
Once the plate is off with the foam attached you can re-use the plate and foam straight away, unless you are going to the other brand side (AMD to intel or vice-versa).