Ahh, I see you have used a List style and applied it to Heading 1 (so Heading 1 list's settings are effectively being overridden by the list style). I never use list styles because of the extra layer of complexity, but since you have used it, the code needs to be addressing that ListTemplate instead
Code:
Sub ScheduleUpdateStylesFromTemplate()
Dim x As Long, aLT As ListTemplate
Set aLT = ActiveDocument.Styles("Numbered Headings").ListTemplate
x = aLT.ListLevels(1).StartAt
ActiveDocument.UpdateStyles 'reimports styles from the attached template
aLT.ListLevels(1).StartAt = x
End Sub