R134a fitting installation

[Reddeye]

I’m slowly replacing and updating parts to get my AC working again, and purchased an R134a fitting retrofit kit. Couple of questions for the experienced:
(1) It came with two high-side fittings (different thread sizes) and one low-side fitting, is this to account for different sizes of high-side sizes or is there a third fitting somewhere in the system?
(2) There doesn’t seem to be a way to remove the old fittings. Do the adapters just screw on to the existing fittings, and I just need to remove the Shrader valves from the old ones?

[truckster]

There should only be two fittings. You don't need to remove the old fittings, or even the Schrader valves (at least with the adapters I've seen). Just make sure the old fittings are clean and install the new ones over them.

[CWW]

The systom I converted to 134A the the adapters just screwed on the existing fittings and I did not have to remove the Shrader valves from the old ones

[68Gold/white]

The fittings I have used, required removing the old Schrader valves.
Some older cars used the same size fitting on the high and low sides, and some used different size fittings. I'm thinking the kit was made up, to fit many applications...

[Reddeye]

I think the kit I bought requires the old Shrader valves to be removed, because the bottom of adapter center pin touches the top of the old center pin and prevent the new valve from opening.