Do you have the solution files to hand?

If so make a backup of them in their 2005 state, then open the originals in BIDS 2008, it should run you through the upgrade process.

Obviously if there is any errors they will need fixing but if not, then you can simply just redeploy the solution to the 2008 reporting server.

Also take a look for a tool called RSScriptor if you don't have the solution files, it will pull the RDL files from 2005 and then you can open them upgrade them in BIDS 2008.