This is the best topic I found to talk about Shield2Gold issues.
I have been testing the AI with gold upkeep for some time (I find it very important to enjoy this game), and I always thought that the AI does not notice when his economy is breaking due to amount of units supported by gold. When I use Shield2Gold rule, I see continually AIs that lose his buildings and units due to low funds, when they could have avoided it by just increasing a bit the tax rates. I have not checked the code, but it looks like if AI is someway blind to bankrupt caused by Shield2Gold units.

As cazfi pointed, it is possible to get a similar behavior (units changing from shield to gold upkeep under certain governments), by using "uk_shield, uk_gold and effects Unit_Upkeep_Free_Per_City and Upkeep_Factor", instead of Shield2Gold.

And my surprise was that the AI seems to manage perfectly the economy with units supported by gold when I use uk_gold instead of Shield2Gold.
I'd suggest to some coder to verify that units with uk_shield and Shield2Gold are taken into account by the AI the same way than units with uk_gold.

Tweak which units have Shield2Gold in experimental ruleset.

See gna bug #17726.

(Browse SVN revision 19510)

I think units uk_shield, uk_gold and effects Unit_Upkeep_Free_Per_City and Upkeep_Factor already model everything I see need for (setting upkeep for all units in shields under one government and in gold under another government is not even close to limits).
So, question "Why Shield2Gold exist?" is good one from my point of view. I really don't understand what we try to model with it. Or is it used purely for game balance reasons?
README.ruleset_experimental: "After The Corporation is researched, the upkeep for most modern military units (Musketeers onwards) is changed from shields to gold"
Why is upkeep of these modern units in shields before The Corporation is researched? Why only these units change, and not all?

The last two comments are both persuasive. However, the fact that two smart people came up with two [quite different] solutions suggests that we need to revisit the reason Shield2Gold exists. I would suggest that the root motivation is that in the real world, 'modern' units are not supported by individual cities, but rather by the central government. In fact, it's rather common for units now to call their "home" some remote base that only has a small village nearby. I was stationed at such when i was on active duty.

The other aspect is that more technologically advanced units will necessarily require exotic supplies that are not easily produced in every factory. How many town could really build their own Stealth Fighter? I'm not sure if we should change this effect to work by government type or stick with unit type. If units are the deciding factor, then i suggest that units supported by Shields should no longer be effective in a level of technology that corresponds to our year 1945.

Well, I'm not sure what the historical basis for units taking production (and free upkeep) is -- sketchy research on Wikipedia suggests that even ancient armies consisted in large part of paid professionals -- so I don't think we can use that.

I'd be tempted to start at the modern Riflemen/Cavalry/Artillery (so Musketeers/Dragoons/Cannon are the last "free" land units). For ships, start at Ironclad (Galleon/Frigate are "free").

All military units beyond that (including all air units) has Shield2Gold, with the notable exception of Partisans (as in current rules -- I think this is appropriate).

Wrt the current experimental rules, that boils down to moving the Shield2Gold flag from Musketeers to Riflemen, and adding it to Cavalry.

(file #12512)

Also, neither Dragoons nor Cavalry have Shield2gold set, although they're more or less contemporaneous with Musketeers/Riflemen.