Not exactly, you can switch at any point, it is much easier straight after a 'checkpoint'. It involves a bit of admin, they have to request your training records off the old school, and will probably mean you have to do a number of revision flights to get you up to speed especially if you are changing aircraft type, airport or whatever.
It's definitely worth putting in a bit of work now to get a school that you are fairly sure will see you right. If you don't like your current instructor try changing that before changing schools!