Ok, I understand the context of "custodian" now. I thought you were talking about custodian of the 529 account, but apparently you were talking about custodian of the student in terms of FAFSA.
I don't think there are any gift implications if the 529 account ownership is changed, as long as the beneficiary isn't also changed.
Something to think about: as the non-custodian parent for FAFSA purposes, if you retain 529 ownership and can wait until your child finishes the first semester of sophomore year before using funds from the 529, you can avoid any financial aid impact. (This assumes that your student will start college in a fall semester and finish in four successive academic years.)