do i need new rotors if i upgrade my disc brakes?

stuisnew Posts: 366
edited April 2011 in MTB general
Probably a dumb questioon but couldn't find it asked before on the search.

I have Hayes mechanical disc brakes at the moment but am going to upgrade to 'proper' hydraulic discs, maybe the shimano deores or clarks skeletals.

My question is, do I need new rotors? For example, merlin have some really good deals but excluding rotors, so if i can use my current ones i'd be a happy chapyy!

Any help appreciated!