I got the same mobo lol and the same thing just happened to me (last night!). I had to remove the whole mobo, as the clips are on the back! There might be a way to break the little plastic clips, but I didn't wanna risk cracking/scratching the mobo, so I just took the whole thing out. Then, from the back of the mobo, get a pair of cutters, and cut both of the plastic clips (just keep cutting away at them until u can pull the fan off). Then clean the old thermal sh!t off of the northbridge, put some new thermal paste on, and install your new HS! Good luck, and if you have any questions, write back, PM, or email me.
-Josh