Actually you will do that yourself...
When you setup your my Disney experience account you can link all of your tickets into your
MDE account. You will also link your resort ressies and dining ressies. Once you have those linked, you can order your magic bands and make your FP+ on line via your MDE account.
When you check into the resort, they will scan your magic bands to make the MB's your room keys.
Now your MB's will be your room key and have your park passes and dining reservations ..all in one.
Once you link your park passes to your MDE account(this would include your old park passes and your new ones) I believe you can select which passes you want to use first. By doing this you will avoid the problem I had back in Feb which entailed numerous e-mails and 4 phone calls with various cast members when the "system" used the wrong park passes from my MDE account.