Hi @keesscherer,
This is perfectly possible in APP with your data.
(I calibrate, all my own data with flats of different iso or gain compared to the iso/gain of the light frames )
Since you indicate that you still have the light frames and flat frames, you only need to make sure that the bias pedestal is removed from both light and flat frames using the correct bias/gain for those frames in the calibration proces.
So using an iso 1600 masterbias/dark to remove the bias pedestal in iso200 flats, is fundamentally wrong. You need to apply an iso200 masterbias/dark to remove the bias pedestal in iso200 flats.
This rule in the APP calibration engine actually prevents user errors and doesn’t pose limitations, since bias frames are easily made. Even years after you took the original data for most camera’s.
What would need to be “overridden” in your opinion?