If you're changing to a new host/domain then your registration will need to be updated. If you're just changing the directory part of the URL then your registration is still valid. For example, if your registered using the URL
http://www.mymmldomain.com/mml/mml.html then your registration is still valid if you change that to...
http://www.mymmldomain.com/new_mml/mml.html
or
http://www.mymmldomain.com/mml/mml_new.html
Your registration would need updating if you changed to...
http://
new.mymmldomain.com/mml/mml.html
or
http://www.mynewmmldomain.com/mml/mml.html
If you need your registration updating, just send a PM to 'administrator' containing your MML username and details of the new URL.