Or whatever it's supposed to be called!
I can certainly see continuation of preferential interviews, but a flow-thru I can't see the MEC signing off on something like that.
I believe they give you a form to sign on the first day of class, they send it to your previous company, and then get a confirmation back.
Whether your previous company actually takes away your seniority is up to them (I've heard of some airlines telling XJ that you no longer have seniority, but actually keep your number on the list).