I Have Been Taking The Micronor Contraceptive Pill For A

I have been taking the Micronor contraceptive pill for a few months and have now been re-prescribed Microgynon. My doctor advised waiting until the current POP pack was finished before starting the combined pill, but advice on the info that comes with Microgynon says to wait until the first day of bleeding. However I don't have regular bleeds on my current pill, and I don't understand why I have to wait until the end of the packet to switch - I appreciate that some pills have fixed cycles with real/placebo pills, but since I've been taking Micronor every day for several months, how does my body know whether I've taken 7 or 28 days of my latest pack? What will happen if I switch early? Any guidance is appreciated.