How hard would it be to go from 14 series(reserves) to 12B(Combat Engineer) Active Duty as far as job availability?

Most 14 series and 12B's are generally listed as balanced MOS's. You need to contact your Career Counselor, who would have the latest and greatest on the IN/OUT's and the availability of class seats.

What are the in/out slots looking like for your specific MOS?
What about 12B? A recruiter is most likely to have that information.

Once you have those questions answered the difficulty will come more from the general process of transferring from reserve to active duty.

Also, because seats for MOS's are changing every day, when you get to MEPS to swear into the active component your 12B may not be available that day.