1. How does the powder measure work when you want to change calipers; the RCBS powder measure is threaded into the cast iron press and it is not a part of the removable tool head; right?
Depends. If you are loading rifle then yes, the powder measure is usually in station 3 (the stationary station in the press casting.)
Only the mic setting and possibly the depth of the powder die in the casting needs to be adjusted a tad....empty the hopper by removing the thumb screw and unlatching the spring...pull it out and empty.
You can also use station 3 with pistol, but I find it more versatile to buy a powder-through expander for each pistol caliber, so you can mount the Uniflow in station 2 (on the removable die plate). That way the press sizes/deprimes in station 1, primes, expands, and charges in station 2. That leaves your stationary station 3 for a lock-out die, and the last 2 stations to seat bullets.
If you choose that route you may want to buy additional powder dies and lower linkage brackets to permanently mount to each of your pistol die plates (tool heads). Then all you have to do to change calibers is unscrew the linkage screw, unhook the spring and lift off the Uniflow upper. Dump the old powder, and remount the Uniflow upper onto a new caliber tool head. Only thing that remains is turning the mic to the recorded spot for the new powder/caliber.
The Uniflow parts for pre-installing on tool heads look just like this Hornady product called the "Quick Change Powder die" ($25). RCBS only sells the parts separately...powder die and lower linkage brackets & screws. (lest I confuse you, I use the Hornady picture because it illustrates the parts....Hornady P.M. and RCBS Uniflow are dang near identical, but parts are not quite interchangeable. (that is you can't use H. expanders on R. powder dies and vice versa)
Yet the Hornady press-activated linkage, powder dies, & expanders
together will fit the Uniflow, but that isn't a necessary option anymore since RCBS delivered their own new & more versatile powder-thru expanders.
What is included in the caliper conversion kit; does this come with a new shell plate or just the new head?
There's no "kit" as such. Just buy a shellplate (if needed) and a die plate (tool head) for the new caliber. Some day RCBS may offer "quick-change kits". Such, for pistol would have the two plates, plus a Uniflow powder die, expander, and lower linkage brackets & screws. But then you'd have an extra shell plate if you have other calibers using that plate already....so maybe no kit is best. I would like to see them offer a "quick-change powder die like Hornaday does though...would be cheaper and easier to order.
Primer change is just unscrewing one size primer rod and screwing in the other size...less than 10 seconds.