Now that you are starting the progesterone-only pills, which are taken continuously without a week of placebos, your previous veset kavua no longer applies. You will revert to observing the usual three onot (chodesh, haflagah, onah beinonit). This first month you will observe only the yom hachodesh and onah beinonit based on your last period, but not the yom haflagah which was solely based on the pill's schedule. After your first period on the minipill you will begin to calculate all three onot.
In practice, you may experience irregular bleeding or spotting over the first months of use. Take precautions (outlined in our article on stains) to avoid becoming niddah unnecessarily from staining. If you do find a stain that makes you niddah, it will not affect your veset calculations.
Over time, some women stop getting their periods while on progesterone-only contraception. In general, once you have observed all three onot from your previous period, you will have no more onot to observe until after the next time you menstruate.
Please feel free to get back to us with any further questions.
B'Hatzlacha!