you should be fine, up until recently all i ever did was replace some ram, change a hard disc or change a Gfx card.
But within the last month i decided to buy a new PSU and replace the old one, add another Fan to my case and replace the heatsink that was on my processor. the last one required me to completely remove my motherboard from the case, then remove the old heatsink and bracket, remove the processor and clean thermal paste from it, all before adding a new bracket more thermal paste and attaching the new heatsink and then putting the enitre system back together again.
this of course didnt come without its problems, the heat generated inside my case caused the screws that were holding the motherboard in place to be really really tight, i had to use a screw driver plus a set of Plyers to remove them, but it was all straight forward and worked fine in the end.
one thing i would definatley reccomend is to have your motherboard manual with you as you will need to connect the harddive led, power button, power button led, reset button etc to the motherboard from the new case.