For all I know, TTRS has four or five distinct eras.
Yes, there are different eras: theorically 1920-1950 (actually 1930-1950 in the code), 1950-1980, 1980-2010, 2010-ever. But it is still a little bit more complicated. In the original version, some houses are available to build in the time period 1920-1980, other only until 1990 (the hospital), etc.
Or is it solved in TTRS such that the house IDs are re-used and only different graphics are displayed, depending on the construction date?
No: each house has its own ID. So, you have differents ID's for houses in the different eras.
Did you also change the introduction date for the houses which are supposed to be part of the later eras?
No. Here is what I *exactly* did:
- I firstly selected all the houses that had as starting date 1930 (low byte 0A in the property 0A of action 0). So, only the houses which started in 1930 are affected by my changes.
- For each of them, I changed the introduction date (using long format dates: feature 21), giving for most of them 1800 as starting date. For the ending date I followed the original date. For example, a house which had as starting date 1930 and ending 1950 (0A 0A 1E in action 0), it becames "21 \w1800 22 \w1950". For a house that had as dates 1930-1980 in the original (0A 0A 3C in action 0), I changed it in "21 \w1800 22 \w1980". Obviously, I also had to change the field 3 of all the affected actions 0, since they define the number of feature, and the change implies adding one feature (the long system date needs two feature, while the old system needs only one).
- For some of those one, I decided arbitrarily to give a different starting date as 1800, to ensure some historical coherence. The Lenin statue, for instance, had (originally) as starting-ending date: 1930-ever (0A 0A FF). I changed it to 1921-ever ("21 \w1921 22 \w9999"). Same thing for Firehouse (there are trucks in the picture ...), for some high building, for the kino.
In the results, there are far enough houses (including hotels, libraries, hospital etc) starting in 1800 to have a coherent (and pretty) game, even when playing only with TTRS houses. Progressively new houses become available in the XIX century (the Stock Exchange around in 1850, modern buildings, firestation, cinema around 1900, etc). For the time after 1930, everything is just exactly as it was in the original.