Basically you change the start date in defines.txt in the common folder (but you should make a small mod and change it there in stead).
If you want to go all out you can also change it in bookmarks.txt but that is only cosmetic and has no effect on the game as such.
The historical characters will only show up if you adjust their birthdays etc. in the respective history files to account for the change you made in defines.txt as mentioned above. However the game will generate random characters for you so if you can live with that the only change you need is the one in defines.txt.
Okay, thanks.