Start menu - then select "RUN" - Then type Regedit, then hit enter.
and navigate to "HKEY_LOCAL_MACHINE\Software\Paradox Entertainment
Then the EU2 folder, look for the path command and change it to the new path.
If you do not understand the above then do not do it, messing with the registry can mess your system up if you do something wrong.
What you can do is wipe the game from your system, you will have to manually uninstall since you moved the folders around, or move them back to where they should be and uninstall. Then reinstall and select the new folder you want to play the game from. If you do decide to manually uninstall the step by step instructions can be found in the self help guide sticky thread.