It's my understanding that the belief in Tibetan Buddhism is that when a Lama is reborn he (not meaning to be sexist, but I know of no female Lamas - please let me know if I'm wrong) becomes that same Lama again. From a western viewpoint, it might be said his soul is put into a new body and he returns as the same Lama in the new body.
However, recent comments by the Dalai Lama have me wondering about this. It seems he wants to be reborn in order to continue helping others, but without becoming the Dalai Lama again. (It has been speculated that there may be political motivation to this, as the Chinese want to designate the next Dalai Lama.) However, wouldn't the 14th Dalai Lama become the 15th Dalai Lama if he is reborn? Would a Lama be able to end a series of Lamas even though he is reborn? (I have heard of Lama lines ending, but it's my understanding that the belief is that the Lama chose not to reborn.)
Or is my understanding here completed off base?