If you open the report created in 2008, while in 2008 R2, are you prompted to run through a conversion? If not, I don't think you need to convert. What you want to make sure though, is when you deploy, you are targeting the correct server version.

From the Solution Explorer, right click on the top level item and select Properties. Last item will be TargetServerVersion.

Check this also. http://social.msdn.microsoft.com/Forums/sqlserver/en-US/0aaff988-7bff-40b8-82f4-ab3b8f1ad4f7/ssrs-2008-r2-compatibility-with-ssrs-2008-questions