For "full disclosure", it isn't hard to do a caliber conversion on a 550b.
Assuming you have a separate toolhead and powder die for each caliber, you set up the dies in that toolhead and generally leave them alone. Unless you are setting up for a new bullet, and then the seating and crimping dies will need to be readjusted.
You move the powder measure to the powder die and set it up for that caliber. You might have to change from large bar to small bar, and then you adjust for the caliber you are going to load until the thrown charge is right.
You probably will have to change the shellplate and locator buttons. There is a chart in the back of the manual that shows which numbered shellplate goes with which caliber. Sometimes you'll get lucky and the caliber you are changing to uses the same shellplate/locator buttons. If so, you can skip this until you change to a caliber that uses a different number.
You also might have to change the primer assembly from large to small. This means swapping the primer slide and magazine tube. Dillon supplies both assemblies with the standard 550b, so you don't have to buy anything to handle large or small primers. And, of course, there is the nuisance of emptying the magazine tube if it has large pistol primers and you are switching to large rifle primers, for instance. (For this reason, I try to reload in multiples of 100, so I always leave the magazine tube empty.)
Changing the primer assembly over is the most time-consuming. The quick-change of the toolhead doesn't help with this at all. I've read of some folks, who must do an awful lot of reloading, buying another 550b so they can have one set up for large primers and another for small.
This seems a bit extreme to me, but I have carefully avoided buying any guns that use small primers so I may not really understand the situation...
