I recently had to downgrade a server from Enterprise to Developer as we wanted to ensure the proper licensing compliance on a dev box.

What I did was:

1.) Clone the server using VMWare (all our dev servers are virtual, only production servers are physical).

2.) Rename the existing server "servername_old".

3.) Detach all the databases on the clone.

4.) Uninstall SQL Enterprise on the clone.

5.) Reboot.

6.) Install SQL Developer on the clone (we also had the "old" VM to use as a reference.)

7.) Attach all the databases.

8.) Script out the creation of the logins from the "old" VM.

So, in less than 2.5 hours (which included the time it took to create the clone) we solved a similar issue with a dev server.