D2.11 states it all. ".....A VCA can also be changed following a successful Motion Attempt (2.401), as a result of firing outside its CA during any fire phase (other than its own Mph; C5.1), or at the end of any fire phase in which it is still eligible to fire a turret/bow-mounted weapon (3.12 and C3.22)."
Also, see the example of D3.51 which states "...He decides not to fire his BMG because to do so he must change his VCA and that would require penalizing the first shot of all CA-restricted weapons firing as NT Gun Types..."
This confirms that changing VCA during Prep is allowed.