What is required to 'upgrade' software from DAL C to DAL A?
The main difference in effort is in the amount of verification required.

Design Assurance Level (DAL) C only requires statement coverage whereas DAL A requires MC/DC coverage.

Some of the DO-178C objectives have to be meet with independence where they didn't need to be previously, and some life cycle data goes from CC2 to CC1.

I would suggest looking at the objective tables A1-A10 in DO-178C