I hope you don't have the trouble I had
Rear subframe removal and bushing replacement
There are loads of other threads on this forum - do a search!
If the main bolts are stiff to get out I recommend trying to take the weight off of them when undoing by lifting the trailing arm a bit.
I have read (here) that you can replace the mounts with out removing the springs - I guess it is possible but I think it is a bordering on the unsafe as it is quite easy for the car / sub frame to move and thus making lots of potential problems! The main bolts are often quite difficult to get back in - I think it is a way easier job to do without the added influence of a spring.
For the record I've now done the job twice. It takes a bit of time the first time - but as with all things it is quite simple once you've done it before. Good luck.