You have to make a critical choice: stay with your girlfriend; walk away from this particular apartment of girls entirely; or make the switch to another roommate. One school of thought would say that as soon as you date a person, her current roommates are removed from your dating universe — at least until the girls are no longer roommates. Only you can decide how to proceed because only you know the intricacies of your relationship and your personal needs and desires.
You will probably want to consider, though, how serious your relationship has been. If your girlfriend has been expecting a ring any day now, your chances of making a successful switch are slim. Likewise, if the roommates involved are inseparable best friends, you’ll want to consider the stress this will add to their friendship and to your new relationship.