Hi there Lou
I had this very question come up when I was attuned. My master felt strongly that one should return to the master who attuned them. However she felt that there where always circumstances that would make this a hardship. In her opinion, when a change of master was desired, the person who was seeking attunement should talk to the master and explain her/his desire to change.
I am now a master/teacher myself. It is my opinion that a person should seek attunement from a master they are "comfortable" with, and with whom they feel a "connection". I truly believe this should be about the person seeking the attunement and what is best for them. So, in my own case as a master.. I do not mind at all if someone I have attuned, seeks further attunement from another master.
Good luck....love, light and laughter...peg