Short of someone upgrading an older K-80, the latest FCG upgrades on the guns started on serial number 25000, being the 1998 models and later.
Next comes if you are trying to piece one together or not, and with a receiver going for $3k upwards, and the fact that you can buy a new Pro Sporter for under $9K, unless you can find the barrel you want on the cheap, better to either buy a complete used or new gun than to try to piece one together.
Lastly, unless you find a gun that fits you out the gate, buying a woodless gun and have a stock made to fit you will save you money in the end as well.
I'll give you an example of this since I am building up a new Pro skeet ($9700 with tubes and Americase without wood, all brand new with warranties).
The forearm I need is the straight front skeet 3iii (wider forearm than stock). In the butt stock, parallel adjustable comb at 1 1/2" drop, Lop 14 7/8", heal 3 1/6" drop, 1/4" cast off at heal, 1/2" cast off at toe, and 1 degree pitch at pad. Besides K having the forearm, the butt stock would need to be a custom order from them, and several thousands over what I can have the entire stock set made here in the states from a stock maker with the same grade of wood or better (read stock is costing me just under $2K in fancy french walnut fitted).
Next,
You are going from a Italian gun to a German gun, which have a totally different feel. Before you make this move, spend some time in a K gun to make sure that it's the gun that you want to go to. Not only are you going to a totally difference feeling gun, but from a low rib to a high rib on the ACS double barrel as well. If he feel is not to your liking, then
the P guns will be more of a like feel as the Beretta gun, and regarding a High rib P gun, P makes them as well (MX-10 in used, and adjustable high rib on the new receivers) if you find the feeling of the K gun not to your liking.