Update from 3.5 FMS to 5 pro

Hi,

     we have a couple of streaming interactive servers that are 3.5 and 4, do we need to purchase the full 5 pro licence or just an upgarde one?

    Also are there any issues with running the update process, or should we do a clean install?

   Finally do we need to backup our videos (these are located in an apps dir outside of the main adobe dir) and how do we roll back if needed? Sorry for all the questions! Any advice would be appreciated..

Many thank Daran