How can I change my USAR MOS?

(I am currently Pre-BCT but stil answer i am willing to wait 6 more month if need be) I'm stuck with an MOS I did not want or even know about until I was at MEPS, I have spoken with my 1SGT but he of course being in my company siad no chance, i need an unbiased, but intel. and honest answer.I asked for 61H or 31B and was told "I'll see what i can do" and being told nothing at all prior to MEPS, i was stuck with 74D and the rec. told my MEPS instructor I personally requested this, I had NEVER even heard of 74D until i was signing. PLEASE HELP!!!