Currently we are using LLBLGen Pro version: 2.0.0.0 to generate the entity class from SQL Server 2005. We are planning to move to SQL Server 2008 R2.

Will I need to re-generate my entity, relationship etc classes. Are there any known issue while migration from 2005 to 2008R2

SQLServer 2008 is supported since v2.0 so you are ok with that. You wont need to regenerate code. You just need to set the compatibility mode (wich is the same as SQLServer 2005. more info.

Thanks a lot for this quick response. Sorry to ask this but during my further investigation I realized that one of the application that need to be migrated to SQL Server 2008 R2is using entity classes generated using "LLBLGen Pro version: 1.0.2005.1"

Will I be needing to re-generate the entity classes for this application or can this work too by merely changing the compatibility mode ?

Still there is no need to regenerate the code. The generated code from 1.0.2005.1 works fine on 2008.