They have exactly the same function: the valve opens a port between the pump and the dampers when the ride is too low, opens a port to return/reservoir when above ride height, closed when at ride height.
The T124 valve has a port for each side, not sure whether the T123 does. If the same amount of ports, the same threads, they are interchangeable provided you are willing to make your own brackets and possibly modify the lines to the new locations, this alone makes it a pretty big project though. Then you will need to fabricate a new link rod (adjustable preferrably).
Operating pressure should be the same, and the same fluid.

interesting project, I'd expect it to go the other way though since the T124 valve is supposedly non-rebuildable and the T123 valve has a rebuild kit available.

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- Jeff