Family Second Preference (F2) change into Family Third Preference (F3)

[mysaes_jgo]

My girlfreind's father fill her petition about 10 years ago While he was Resident.

Sep-2009 some documents for the Choice of Address and Agent Arrive.
Oct-2009 Her father Paying Fees to the National Visa Center (NVC) US470.
Jan-2010 Her father become Citizen.
Mar-2010 Her father send his Citicen document to (NVC).
JUl-2010 (No yet come) I WANT TO MARRY HER. SO ON THIS CASE will change category to (F3).

Is this posible knowing that already pay NVC fees and NVC already assign a case number.

Please advice. We want to know if we can marry or not.