HP12c additional principal payment in amortization?

[caccorsi]

Hi,

Does anyone know how (or if it's possible) to calculate an amortization schedule with additional principal payments on an HP 12c?

For example.
PV = 100,000
n = 30 x 12
i = 3.0 / 12

Solving for PMT = -421.60.
I'd like to add an additional -500 to the PMT then solve again for n so see how quickly the loan is paid off.

Thank you