I can state again to clarify. Your choice of Government lasts the entire Age and cannot be changed.
One feature discussed for the Modern Age has been Ideologies, and given that apparently (from the Mexico thread) a Firaxian confirmed Governments are a one time for the whole Age deal and given that the Modern Age seems to start sometime around 1650-1850, that seems difficult to fit the "Big 3" ideologies in.
However, I had a thought about how it could work... separation of "Governments" and "Ideologies"
after all in WWII most of the combatants had similar forms of Government
"Authority" Japan was a constitutional Monarchy, just like "Freedom" UK
"Order" Soviets had elections for their governmental bodies (one party state), just like "Freedom" civs
Perhaps... (unless Mexico) in the Modern Age
First you Choose your Government (permanent with it's various Celebration bonuses
Constitutional Monarchy
Presidential Republic
Parliamentary State
Then early in the age you can also choose an Ideology (Authority, Order, Freedom)... each of which has its own civic tree (like theology in the Exploration Age)
This ideology affects diplomacy (both with civs and IPs), happiness, which social policies you can put in your government, and victory conditions
And you can change it.... at a significant cost
but the Government stays the same.... you still have an elected President... but who can run, who can vote, and who counts the votes might change based on ideology (and the social policies that go with them).
Thoughts