1) Firstly collect all the tools, at least freewares that can help you in your migration analysis. SQL Server 2008 R2 ships with SQL Server Upgrade Advisor, which can be the best starting point. This tool is also a part of the SQL Server 2008 R2 Features Pack. You can learn more about the same from here. This tool covers all areas, right from database engine till SSAS.
2) When you start your design, you would have to make a clear distinction between whether you want to perform an in-place upgrade or create a new instance -> deploy solution on the new instance -> ensure synchronization between old and new instance -> abandon old instance. Check out this article for some more info.
3) You should keep in view where you plan to do the upgrade, i.e. on the same box, in the same domain, or across different servers and different domains. This would throw up the challenge of security configuration.
4) Environment configuration needs to be planned for each service separately. For example, SSIS packages can be expected to use configuration settings from different sources like environment variables, configuration files, database and other sources. SSRS configuration might reside in config files for reports server as well as reports manager. Virtualization is the key factor is testing all such scenarios.
5) Finally the biggest risk factor needs to be calculated, i.e. identifying the right sampling to test on the targeted edition. SQL Server 2005 came with it's first mature BI offering. Several components of different services have undergone architectural changes, several features are discontinued, several features have behavioral changes and several features are guaranteed to break when migrating from lower to higher editions.
a) Deprecated Features in SQL Server Reporting Services
c) Breaking Changes in SQL Server Reporting Services
d) Behavior Changes in SQL Server Reporting Services