How hard/expensive is it to switch out gearing on an A-body?

As far as swapping pigs goes, first find out if the rear axles still have OEM adjustable axle (wheel) bearings, or if they have already been replaced by "green" bearings. If you have the OEM adjustable bearings, keep in mind that the pigs must both have the thrust button in the center. And every time you swap pigs you'll be adjusting those axle bearings. If the axles have green axle bearings, then both pigs should not have the thrust buttons.

If you plan on swapping pigs a lot, install a drain plug on the bottom of the axle housing.

And, invest in a transmission jack. Those pigs are heavy!