Instead of using now() each time. Set in Open Rules if the form is new (a required field is blank or whatever) to populate a field with Now() which essentially is creationdate.
Then use this new creationdate field in your file name and do not modify if the form is opened again. That way your file name remains the same through out the life of the file.
Another method would be to put a file name field in the form, use your rules to create the name if the field is blank, then use the field as the file name. That way it is created once and gets reused each time.