Congrats man, I'm enlisting next year. I was really considering 12B but decided I'm going to try to go for heli pilot, however chances are by the time I get there next year (18th B-day is in may) all the WOFT positions will be full, so I'd either have to wait until next year or go try to go in as another MOS for a few years until I can reclass and hope a spots open. If I can't initally get into heli's I should be signing as a 31B 
Conrgatz again! 
You can hope in one hand and crap in the other.
I strongly suggest NOT doing this.
Waiting for the position to open is what you should do, regardless of what they try to talk you into.
Did I mention that it is a bad idea?
You are not obligated to take what they have if you have qualified for a MOS that is not open when you want to enlist. Their job at MEPS is to fill open slots with warm bodies, not to give you what you want and they will say, and/or try to coerce you to take what they have in any way possible until you relent and agree.
You can tell them "Thanks for the offer, but I think I will wait until the job that I want is available before I commit X years of my life to you" They know how to get in touch with you.
Switching an MOS is about as easy a finding a bag full of chicken teeth...