How to change the SID of webforms to increment from a new starting number
If it's a Drupal 7 site: 


Edit the webform.   
Go to the "Form settings" subtab of the Webform
tab.
Scroll down and expand the Advanced Settings section. 
There's an exposed field for the Next submission number.






If it's a Drupal 8 site:


Edit the webform.   
Go to the "settings" subtab of the Webform
Scroll down and expand the Submission Settings section. 
There's an exposed field for the Next submission number.