Once the barrel is fitted to the frame by the factory, it's just a matter of removing the forend screw then the forend, open the action and remove the barrel, install the other barrel, close the action, install forend and screw. Headspace is set when the barrel is fitted, doesn't change when changing barrels. They're plug and play once fitted.
Modern(made after 1998) SB2 frames are heat treated, that's the difference between the high pressure SB2 and the low pressure SB1 frame, for the most part.