What I say here apply to all AGE engine games, so I hope the RUS team won't mind I intervene for once
First, a demo date can't be changed (well you can but it will does nothing!)
Second, the script*.ini files are only used when someone generate a scenario, the output file is a .scn, i.e the scenario file you see in the New Game menu.
If you want to generate the scenario anew, it is possible with the full version. The alternative being that you hack directly into the .scn (which is also a text file) the end turn date.
There is no special problem in postponing the end date. The game anyway allow you to continue after the official end in the full version.