The Army does not work that way at all.
You pick an MOS, and that is what you get. And you are sent to that school and trained in that MOS as that determines where you go to Boot Camp. It if is the 11 series, you go to Benning. If it is for the 25 series, you go to Gordon. If it is air defense or artillery you go to Sill. And they are not going to waste money training you in one MOS, only to at a later date send you to be trained in another. Nobody who has a contract saying they will be a 25X gets sent to Benning and trained as an 11B, only to then be sent to Gordon later in their career to be trained as a 25B.
As far as I am aware, about the only service that regularly offers "Open Contracts" where you are assigned an MOS purely at the needs of the military is the Marines Corps. The Army has not done that in decades.