Bit like leaving home in the convertible, getting halfway to work and wishing you could "trade" for the 4 door sedan because it looks like rain, isn't it? Its not in the least bit realistic in any way.
You really want to switch, fly back, end sortie and up the other one.
Once you start a sortie, your locked into it until it ends by your death, bail, landing etc.
FYI if your score means that much to you, that you'd wish to change the game so you could switch.
I suggest you have a problem.