My opinion is get rid of the metal D loop now. Don't wait.
They are unnecessary weight on the string and don't give you the added forgiveness that a string loop does.
Everyone has an opinion, and there must be enough people that like metal loops since they are still being sold - but not me.
String loops can fail too if they are not tied correctly or they get worn. I had someone at a shop tie one on for me and on my first shot I smacked myself in the mouth. He apologized, retied it. My first shot with the new loop resulted in a second smack in the mouth - I tie my own from now on.
If you want to learn to tie a loop yourself, check this out:
http://www.alseaintegration.com/ptc/string_loop.htm