What's going to happen to me now that the new MOS I'm transferring to is no longer available?

As SFC (Join to see) stated, the 25C is being chopped, likely combined with others to make a new MOS as the military does that fairly often. Here is the thing. You have a contract for 25C. So even with combining it, they still would put you through the training, it would just be amended to become the new MOS. IE you would still attend school but instead of being a 25C you would become a 25U or whatever the new designator is.

I think you misunderstood your contract. There is no MOS reclass on a one year contract. You're on a Try One contract, you try it for a year and if you like it you reenlist for a longer period. MOS Reclass coming from Active Duty is a three year contract. Even if you had chosen that you probably wouldn't have gone to school within the first year.

MOS 25C is being eliminated, and has been on the chopping block for a few years. As a CMF25 you should be aware that several 25 series are being combined or eliminated. In the rlReserve Component several MOS's can fill a slot. For instance several 25 series can fill a 25B slot, and the RC had more flexibility on how many people they can slot into a position.

I would double check with your Career Counselor on this. I'm googling the Hell out of 25C and I'm not finding anything that the MOS got phased out. I mean, I could be wrong, but I don't think I am. What they could be meaning is that there may not be any slots available for 25C in the Reserves ( a little unlikely) or that there are no 25C slots in the area (or near) where you plan on setting up residence.