Hi Jayden,
Magnesh explained in a nice way. Start date in past will be used in backward scheduling and in capacity requirement. If you don't mention any date this will take as current date.
As well automatic scheduling will ensure for any change scheduling will come again. But I think you will not have any problem in your existing order to change the date manually.
For permanent solution you have to change the customizing (OPU3).
Regards,
Krishnendu.
