Other error sources to be aware of:
- Naming conflicts between your registeres user controls on a page and other variables on the same aspx page OR in the user control itself. Resolution: Rename all your user controls in the Register statement, for example, use the 'ux' prefix.
- The language files are not updated automatically, and have to be copied/updated manually.
- Copy all the new template files intot the new project library (aspx, ascx and cs files). Make sure you don't overwrite your own customizations.
- Set the correct namespaces - search and replace - but has to be done :-)
Articles you should take a look at BEFORE you start the migration process: