The top barrel can be sleeved by a qualified German or Austrian gunsmith. Franz Sodia is now out of business, as I understand, but "standard" pre-turned barrel blanks are available from barrel makers. I saw Walter Grass do this several times; sometimes replacing all barrels, sometimes only one, sometimes changing from double shot gun to combination, and once change to combination gun and to sidelock from Blitz at the same time. If the top barrel is sleeved the maker's will be lost, however.
Mike