4
This is what I usually do:
1. Make necessary changes in the General Settings (default theme, default template, etc.). Submit.
2. Delete all files in templates_c.
3. Log out and log back in.
As long as I made sure that no. 1 was done and updated, I usually interchange 2 and 3. This method never fails me. I think if you're still logged in, the system calls for the 'original' theme choice, rather than the new one.
Hope this helps.