This has been my experience with MAs wanting to switch lines into MAC...mind you, this may be an isolated thing, and may not happen in your area, so ask the counter manager at the MAC counter you want to go to if this is status quo for your area:
Here in So. Florida, it is difficult to switch from a line (even Estee Lauder) into MAC...the reason I have heard is that most of the EL counters are part of Federated Stores (Macy's, etc). MAC is not part of Federated. They just rent space at the stores and play by their own rules.
The logic behind it is that Federated does not want their employees jumping ship, so they make it difficult, if not next to impossible to switch to MAC. You have a much better chance of landing at a MAC counter as a freelancer first (they offer a lot of the open positions to the freelancers here, esp if you have cosmetic/and or MAC experience).
Like I said, this may not be the norm where you are...check with someone who might know...