The 120 V pock lights have very short bulb life due to vibration form loading the cabinet bottom they are mounted on.

A 120 V filament cannot be made as resistant to vibration as a lower voltage filament.

Low voltage under cabinet puck lights have very small power supplies rated at 60 W, each capable of supplying power to three 20 W pucks.
The small power supplies can be mounted under the cabinets near the back, and either hard wired to a 120 V circuit or plugged into a receptacle inside the cabinet.

They are not allowed to be powered by the kitchen wall-counter small appliance branch circuits but need to pull power from another circuit.

Yes, changing these fixtures is exactly as you describe. The wiring is different only that line voltage dimmers generally don't work for fluorescents, but work just fine for xenon (halogen) fixtures.

Please note that you're giving up the energy efficiency of the fluorescent fixtures.

Okay thanks for the info. I'm still undecided as to whether or not I really want to change them. But it's good to know what my options would be if I did.