Seems that Kel-Tec listens.
The standard rail is probably a better option for most users, and the ability to install a VFG is just smart.
There's something changed on the rear of the op handle, looks like a design for the bolt carrier similar to the KSG.
It looks like the changes are only in the front end, so I don't see any major changes in the action or trigger group.
At a minimum you'd need a complete new pump handle assembly and a new bolt carrier, which might be the same part as a KSG.
The rail is a drop in upgrade they've offered for some time.
I wonder what the other upgrades are and whether an older gun could be upgraded or if the new design is gun-specific and older guns can't use them.
I'd like to hear what the changes are.