If it is positive and
becomes neutral you can think of it as throwing away its positive charge so the
boson will be positive. This is the case in electron capture.
If it is positive and becomes neutral you can think of it as gaining negative to neutralise it so the boson will be negative. This is the case in electron-proton collisions.
If it is neutral and becomes positive we can think of it either as gaining positive (W+ boson) or losing negative (W– boson in the opposite direction).