SSA/AGPA Classification

Hi all,

Currently an SSA range c since October 2024 (good team and work is manageable.) I interviewed back in September for an AGPA position with "shall consider SSA" and I finally received a final job offer today. They are offering me an SSA, range C position as opposed to an AGPA. My questions are: I assume I can negotiate to get the AGPA position before accepting the final offer? And if I do try to get the higher position, do I go through my prospective manager or to the agency's HR?

Thanks in advance!