Had multiple folks ask this question and there does not seem to be targeted docs as yet in this regard, hence decided to blog on this. Hope you find it helpful.
Ending state: SharePoint 2010, SSRS 2008 R2 Add-in, SSRS 2008 Sp1 + CU#8 Report Server
Ending state: SharePoint 2010, SSRS 2008 R2 Add-in, SSRS 2008 R2 Report Server
For SQL Server 2005 SP2 / SP3 report server in SharePoint mode versions, step 1 (from above scenarios) comprises upgrading to SSRS 2008 SP1 + CU8 OR SSRS 2008 R2 followed by steps 2 and 3( from above scenarios) when upgrading to SharePoint 2010.