Oh! I almost forgot to tell you that you need to replace the thermal paste ( and clean away the old one ) for the cpu when you swap the motherboard. And run the computer for while before you try to remove the CPU heatsink / fan combo. Othervice, you risk that the cpu are kinda glued to the heatsink.
And then it might happend that the cpu will be ripped out of the socket when you lift the heatsink. Arctic Cooling Ceramique or Arctic Cooling Silver 5 are good ones. But be careful getting any of it outside the cpu! Especially AC silver 5 as it's electrical conductive. Just place a blob of the thermal paste in the middle on top of the cpu, as big as a smaller pea. The pressure from the heatsink will spread it in a good way for you.
Also, if you check out the cpu on the down side where all these thin, golden contacts are. In one of the corners they are misssing. The cpu should be placed in the corresponding way into the cpu socket. Make sure that the cpu are all the way down by gently tapping it with a pen or similar before you lock it down with the socket lever and apply the paste. Be very careful not bending any of those golden contacts when you stick the cpu into the socket!