It sounds as if you are not just changing the date of your flight, but also asking Ryanair to add even more bags on this "second" booking.

As I have no Ryanair bookings I can play around with to try and figure out what it is you are seeing in the change process, I'd advise you to get someone there in your household or family to step through the process with you and determine if you are now effectively asking Ryanair to now add another bag for each passenger, and also asking to buy Priority for both passengers a second time.