Probably the right place to ask is my local gun dealer, but here goes:
Antonio Zoli makes both shotguns & combination guns. Is it possible on a standard shotgun to simply change the barrels to shotgun/rifle combination?
Or will some gunshmithing be needed, or is this, indeed, altogether impossible?

When looking at the website I noticed that on the combination gun there is a cocking lever, but as I see it, there is really no need for this as such.
Certainly the Valmet 412S (later Tikka 512S, nowadays Finnclassic) didn't have a cocking piece, it was just to swap the barrels. However, given that Valmet specifically advertised it and no-one else has mentioned it, it occurred to me whether such a barrel swap on the other makes is at all possible.