BK - It's a good job you said "hypothetically" --- BUT - What do you mean by "tennon" & "relief" & in what context?
Every machining operation has a dimensional tolerance & this must be borne in mind.
A "custom" made rifle is made so that all the components ON THAT PARTICULAR RIFLE fit each other precisely. - They are not made to a pre-specified dead size with zero tolerance - even by the same riflesmith. That is just not feasible. Therefore it would be pure luck if you swap barrels that the headspace and all the other critical fits end up perfect on both rifles.
Thread to thread fit is critical as well as head space, which itself is the sum result of the bolt, barrel & action dimensional assembly sizes.
So - get real! get a proper riflesmith to do the job - He can only tell you after the job is underway & measurements are taken, whether any machining to correct any anomolies / headspace issues is necessarry.
If by good fortune no machining of barrels, actions or bolts is needed, then you don't strictly need the rifles re-proofed, although it may be wise to get it done.
Ian