A referral usually consists of a name and a phone number. If you want your previous T to talk to your new T you would have to sign releases for both Ts. You would then be able to tell them what stuff they would be allowed to talk about. Sometimes new Ts don't want to talk to old Ts, sometimes they are okay with it. It really just depends on the Ts. If you want them to then they can, but if you don't want them to then they can't. They can't force you to sign consent documents. Usually there are not letters of introduction, though you are free to say "so-and-so referred me to you".

I hope this clears things up for you.