During gameplay, there is a chance as you approach a 'thresh hold' year for a new 'era' to begin - like bronze-to-iron and iron-to-steel. This chance is small, and calculated by the game engine (so you really have no control over whether it will happen or not). If it does happen, there will be a pop up message telling you (I can't remember the exact wording of the message). If the new era does occur, the game is supposed to switch the bronzeworks to ironworks or the ironworks to steelworks (depending on which era was triggered), but in my experience it never worked quite right.

There is no trigger for chaning bronze coins to paper money. That doesn't happen because they were both used simultaneously in the later centuries. There is also no in-game trigger for cavalry replacing chariots. There is also no gunpowder at all in this game.

Others may add more to this commentary. I searched our forum posts but could find very little on this subject.

it would be cool whit gunpowder and stuff, and what if you reach the year of 2007 a.c.? what then? i don't think you can win whit your cavalry and infantry against modern tanks

A designer recently initialed a bronze age smelters circa 1300BCE to steel age year 100CE(via a continuation mission); the bronze smelters and bronze commodity were removed entirely from the map despite having bronze and iron also enabled along with the steel. The weaponsmiths got a new title 'Steel Weaponsmith'.
So that transition was designer initiated and played out fairly close to what I expected other then the Smelters disappearing.

The questions in this thread are for iron to steel transitions and back to iron, specically designer initiating the iron to steel before the game engine normally checks for a transition date.

Quoted from Gweilo:

the game is supposed to switch the bronzeworks to ironworks

I think that is incorrect as bronzeware is producable until the steel age. Weaponsmiths change their titles and accepted commodity to Iron is all. Lacquerware may appear, becoming the fade for elites, but bronzeware is sellable until the steel age, I think. Bronzeware is dropped, becoming lacquerware as soon as lacquerware become producible or traded anywhere.
My 2 green coppers worth

[This message has been edited by user3 (edited 10-13-2017 @ 11:19 AM).]