First, I should have been suspicious since artillery units have THREE instead of the FOUR elements as in prior AGEOD games with the mechanic - requiring me to have 2 separate artillery units (3 elements from one, 1 from the other) to fulfill the supposed cost requirements, but whatever, that's fine. Just need to do a bit more micromanaging to optimize the low cost artillery to receive those sturdy forts I have no idea how to build.
Then I scratched my head as all my light artillery pieces AND the supply units remain, even after upgrading the forts to the max level 3.
So I do what I should've done the whole time and go to the Fort.str file and looked at the data. Apparently, for once, I am not the one who needs to dig deeper into the "hard to learn, nigh on impossible to master" AGEOD rulesets I so enjoy to play with... Even you sneaky ruelmakers don't know how it all fits together :P.
SO LISTEN UP EVERYONE:
In "Wars of Napoleon" ONLY, forts cost 10 War Supply and 10 Moneys to upgrade, and needs ammo and supply from a supply wagon. No more element sacrifice it appears! ...at least that's what it seems to be. Although there's probably another wrinkle in this I'm missing or just wrong about *sigh*
So stop trying to fit your pea shooting 4lbers on top of your masses of foodstuffs and ammo and calling it a fort you weirdos! Just go pay those workers some money and give them some building materials - they seem like they know what they're doing.

Oh, and when in doubt, trust in Notepad not silly manuals!! I hope this has been an informative post, not merely an echo chamber for my own ignorance :P. Off to play with the new patch!
As a last thought... I am surprised that forts are so drastically cheaper to build and upgrade. I mean, either the cost is missing a zero or two, or apparently star forts are exponentially cheaper & easier to build than a few river gunboats.
??????? that's weird right???