How many miles have you done?!
The rear discs should last for a LONG time as they dont do much!
Anyway, the difficulty depends on how the discs are fastened to the hubs.
If they are just screwed on, like the fronts and the wheels bolts hold them on, then its a simple job.
But...
They could run with the bearings pressed into them (as some Renaults are), which is a pain in the @rse. Youll either need a bearing press (or access to one) to ge the bearings out of the old discs and into the new ones, or buy new bearings and ge them pressed into the new discs.
Not sure about a 172, so whip a wheel off and look.