woah woah woah!
clarify! Are you saying you were a premed and got letters of recs and a committee letter already done and now want to do dentistry? Cause if so...no way should you use that committee letter. If it talks about you as wanting to be a med student, not a dent, you would be a fool to send that to the dental schools.
However, if you are talking about getting a committee letter about how you want to be a dental student from a prehealth committee and haven't gone through the process yet, then yes, you should go for it.