When a gunsmith cuts and rethreads a barrel do they need to remove the barrel from the upper receiver?

The reason I ask is I have an opportunity to purchase a Sig P90/550 with a 20"barrel but the muzzle device isn't removable.
Being a piston gun, it would be a shame to not be able run a suppressor on it.

So I'm thinking of having it cut to 16" and retreaded.
I don't think the barrel is easy to remove from the upper receiver if the process is to remove it during the process.



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