it is not possible
it's allright (Y)
To go a bit more into detai, if you want to switch you need to replace a model file with another model file.
That model file also has to have the right infos, like the place on the human head where it connects.
If you use the hairstyles you can choose for your gender and race you got that all covered.
So you can switch your current hair or a hair dogi with the model file of the hairstyles you have in character creation.
So now for Goku or Vegeta, you have to search trough the models in hope that you find a hair model of them.
I bet numerous modders tried that since the beginning and since no release, nobody succeded.
My guess is that the model of them contains the body and hair together, so you have to try to get the model that far to edit it and seperate it (which only a few can) and then when you got that you still have the problem as baraku said, that you can't get it back in the game working.
Yup, I already viewed all 3d models in this game and as effy said, the hair and the body are not separated and if it is separated, you will have the problem with height. Still not possible in so many levels.