If you are having the withdrawal bleed/placebo week, just switch to the new pill and try to maintain the same week off pills.
For example if you are on the 1st pill & the week before the sugar pills on your old medicine, start on the 2nd pill & the week before the sugar pills. Make sure to wait until you get to the withdrawal week, have the bleed, and start the NEW medications on the last day of the bleed
Essentially, finish your current pack and start the NEW pack after the placebo week on the current pack. If you are skipping the bleed/placebo pills, just switch to the new pill.