User unable to edit a subscription after migration of SSRS from 2008 R2 to 2017
After a week of mucking around with permissions and getting ready to place a support call with Microsoft, Bart Edgerton's final suggestion to me in the Slack SQL Server Community workspace gave the solution:  I needed to give the user permissions to the folder containing the data source used by the report.  

It's a bit odd to me as editing a subscription doesn't allow the user to edit the data source.  But, after a week puzzling over this and tweaking every permission under the sun, I'm at peace with it.

And, finally, this wasn't a migration issue.  At least, not related to my current 2008 to 2017 migration.  The same problem exists on the 2008 SSRS instance.  But, that instance was itself a migration from 2000 or 2005, I don't recall which.