Hiya Roslyn,
Welcome to planDisney! How fun that you and your dad are both going to be at Walt Disney World at the same time. Family vacations at Walt Disney World Resort are so memorable, and even though you will be at different Theme Parks, I'm sure it will still be extra magical to be there together.
The only thing that can sometimes be less magical is coordinating different reservations for multiple people. Although
My Disney Experience makes it easier, I have been in a similar situation as you before. Unfortunately, once the reservation is booked, you can only change the primary party member on the reservation if you cancel and make it again. But your question does give me a follow-up thought. Typically the system will alert you that you have a conflicting reservation and request that you change it before letting you confirm an additional reservation. Still, if you are showing both your father's reservation and yours, it wasn't viewed as conflicting. In this case, there may be no need to call a Cast Member at all. Guests often make dining reservations for other family members, so your father would still be able to go to his reservation without you being present, and you could go to yours. He would need to give your name and, to be safe, have the confirmation number handy. Now, if you are having difficulty making your reservation due to his, you should call a Cast Member at that point. They can help advise you on how best to cancel and modify the reservations so that everything works out "practically perfect in every way."
I hope this was helpful, Roslyn! Thank you so much for visiting us!
Happy Planning,
Ashley F.