Can I get promoted from E6 12b to E7 12a?

Since 12A is an Engineer Officer.....you will not be able to promote into a 12A slot unless/until you get a commission as a 12A

First, do you understand the difference between a 12B and 12A? Being a qualified 12B, I know we were trained to "assist" the 12C. Yes I helped build all the different bridges, but was I qualified to do the 12A job - no. Look at the 12A job description, are you as a SSG, qualified to do this job? If you can show you are qualified and have the paperwork to back it up, then I would say you can take over the job, but if you are not qualified, which I don't know many 12B, SSGs who are then no. The only way I can see it happening is if the gaining unit is willing to accept you and send you to 12A MOSQ, which I have seen this done before in special circumstances, and your case is so. There might also be something put in the promotion that it will become effective upon your successful completion of 12A MOSQ. I have seen this done in other MOSs before.

As an old Engineer Officer, first 12A is an Engineer Officer, used to be 21A. You would have to commission and branch Engineer for those slots. If they are short E7 Engineers, they "might" allow you to transfer pending MOS qualification. I branch transferred to the Engineers as an 1LT 11B and I held a 1LT slot. I could not promote to Captain until I completed what was then Engineer Advanced Course and was "fully qualified".