Well, if you change for example the speed limit in the track rule, all new layed tracks will be layed with ne new speed limit. The existing ones won't be changed if you have modyfied the speed by hand. If not, I'm not sure.
All information about the already layed track is saved in a file called Tracks.bin. You'll find it here --> *YOURROUTEID*\Networks\Tracks.bin
If you drop the Tracks.bin on serz.exe (located in the railworks root path) it will be converted to a editable xml file.
For example you'll find the two speed values inside the 'Network-cTrackNetworkSpeedLimit' blocks.
After you've done your changes drop the Tracks.xml on serz.exe and you'll get your bin file. Be sure you have a backup of your files before you do any changes!
If you tell us what exactly you want to change we can help you better
