You would still use the same cylinder castings if you only pushed the wheels in. Running with additional clearance between the frames when the gauge is narrowed is not a problem. No need to narrow or move the frames in any way. Indeed, since the wheels are the only thing that changes position, as frames, crankpins, connecting rods, valve gear, axleboxes, axles, etc. all remaining the same size and exactly where they are, different cylinder castings would not fit...
A photograph explains:
No moving of frames, changing of cylinders etc. Just moving the wheels on the axle and nothing else. In the case of this model an allen key is all that is required rather than a hydraulic press for full size.
Not on a full size quarry Hunslet, no. There is very little leeway to move the wheel between the inside motion and the frames. Not that it matters, Hunslet didn't design a style of locomotive for variable gauges as did Bagnall and Kerr Stuart, Hunslet would take a design and alter it as necessary and use as much 'existing tooling' if you like to create the new design. That's how we've ended up with so many similar locomotives with lots of variations. For example if you were to dismantle Velinheli, you could fit 4 other shapes of frame and still put it together with the rest of the original loco.
This is something I can't wait to see tackled by Bachmann since there are in the same basic family of locomotives the following non exhaustive list of possible options:
Frames: 5 variants
Buffer Beams: 5 variants
Injector Position: 2 main variants
Safety Valves: 3 Types
Domes: 3 Variants
Boiler Height: 2 variants
Buffers & Couplers: Several variants
Cab/Side Sheets: At least 5 variants
Handbrake Columns: 2 variants
Chimneys, Smokebox Doors, Tanks and associated fittings, lubricators and other sundry details all varied. Very much a case of no two locos being exactly alike.