Change from F1 to F2B

[dcgc01]

Hi, My dad petitioned on my behalf as a US Citizen. The visa category for the petition is 203(a)(1) Unm. S/D - F1-1. I understand that one can possibly change back to F2B from F1 if the category was changed due to status change of the petitioner from LPR to USC. However in my case, my dad was already a citizen at the time of petition. Could you please let me know if it is possible, in this case, to change the category from F1 to F2B while retaining the case number as well as the priority date? Thank you!