jst phone them up when you change the car, they will do a "Mid Term Change" and you will not loose the NCD.
Obviously at renewal in June, phone around and make sure they are still good value. It also might be worhwhile phoning up your current insurers and saying you want a quote on a prospective change of car. If they are too expensive you might want to wait until june for the change of car, and change your insurers at the same time.
As olv says, you will need to pay any increased premioum + admin charges