Good luck. You can submit a 4187 to voluntarily reclass, but getting those approved is rare, but given that both of those MOSs are currently balanced, it's longer than a long shot. You can also try to reclass when you're eligible to reenlist, but whether or not you can do that will depend on what the current strengths of those MOSs are when your window opens up.
If you are in the Reserves, you belong to your unit. You would have to work to find a unit with an open slot for whatever MOS you want and then coordinate the transfer. Typically, this will not happen until after AIT and your unit may require you to spend time in the unit prior to approving the transfer.
Secondly, be aware....there are no Field Artillery units in the Reserves, outside of a couple of training units that are flagged as FA, but are really Drill Sergeants. The chances of you finding a 13B slot in the Reserves are slim to none.