I am working with a customer who (despite my reservations) has set up their system to continue to modify Leads after they have been converted. They are doing this using the Spring 16 technique here:

https://releasenotes.docs.salesforce.com/en-us/spring16/release-notes/rn_sales_leads_edit_converted_leads.htm

1. From Setup, enter User Interface in the Quick Find box, then select User Interface.
2. Select Enable "Set Audit Fields upon Record Creation" and "Update Records with Inactive Owners" User Permissions.
3. Save your changes.
4. Add the setting to your profiles. From Setup, enter Profiles in the Quick Find box, then select Profiles. Select the profile and then
   select Set Audit Fields upon Record Creation.
5. Save your changes.

We are currently relying on this for the operation of an integration we have written for them. If we cannot edit converted leads, then the integration will start running into errors and emails to customers will not go out as planned.

Recently, we found that the integration stopped working in some sandboxes. SF support directed us to this Release Note from Spring 17:

https://releasenotes.docs.salesforce.com/en-us/spring17/release-notes/rn_sales_leads_view_converted.htm

Assign the new "View and Edit Converted Leads" permission to let
marketing and sales operations users view converted leads. Users can
also edit converted leads. This change applies to Lightning
Experience, Salesforce Classic, and all versions of the Salesforce1
mobile app. Previously, users with both the "Set Audit Fields upon
Record Creation" and "Update Records with Inactive Owners" permission
were able to view and update converted leads. Those permissions no
longer grants access to converted leads. To let marketing and sales
operations users access converted leads, assign them the "View and
Edit Converted Leads" permission instead.

In Lightning Experience and Salesforce Classic, users with the
permission can see converted leads in list views, related lists, and
search. In Salesforce1, those users can only see converted leads in
the Recent Leads list.

You can’t unconvert leads.

So, as of Spring 17, the old combination of permissions to edit converted Leads will no longer work and we must use the new permission. Which solves our problem on the sandbox.

The problem is going to come when Spring 17 goes onto our production system. It will interrupt our integration until someone goes in and manually updates the permissions.

I've asked SF support about whether they have a migration plan to alleviate this problem, but it took more than a week and 5 different support agents to get to the first answer. So, I was wondering if anyone has any ideas on how we can make the transition to Spring 17 smoother; or if anyone has the ear of SF and can get a real answer on whether they have a migration plan for this change?