I have a new carb bolted to its own manifold.

The old manifold is fine and I'd rather not remove the old manifold from the head and have rejoin the new one to the exhaust pipe and much around with all the rest.

Is there a good reason not to just unbolt the carbs from the manifolds themselves (at the diagonal plates beneath the carb bodies) and just reattach the new one to the old manifold while it's still bolted to the head?

Not that I can think of. Can be more fiddly and you'd want to make sure you didn't loose any remnants of old gasket into the inlet manifold.

Get some new gaskets and you’re all good. Depending on the carb you might have to remove one of the studs as the bottom might catch the fire wall too much. I’ve got one carb that I have to do this with and one that clears it.

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WD40'll fix it!
If it don't, use a big hammer!