I agree - if you switch from/to compatible flavours by filling the tank with the new flavour over the old one, then you just get a gradual change over to the new flavour over the course of the first half a tank or so.
If you use strong flavours (like Menthol), or flavours that can taste rank when mixed with certain others (certain custards IME) then I suggest doing as djsvapour said and using a new coil for your new flavour, washing out the tank and keeping that coil for the next time you use that flavour.
So, in short - use different coil heads for different flavours, and wash your tank out in-between.