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For the first, the simple solution is to backup all the files associated with the old one, and then edit the backups instead of the normal scenario file. For example, you see 1066_scenario_characters, 1066_scenario_provinces, etc. Backup all of those, rename them (and it's important to remember to rename them inside the file as well), and then place them back into your scenario when finished.
You can create a scenario with a startdate between 1066 and 1453, but you can't create a scenario earlier then 1066 or later then 1453. Unless you know how to edit the .exe file.