Is there a report I can run to identify every item I made nonstock via mass change yesterday?
In order to see changes made to the stock statuses you made, you can use the Audit Report (File > Company > Audit Report). For the Audit File, select Product. For your report fields, only have Special Status checked and/or Whse Stock Flag.
Special Status is the overall status in the Main tab of the Product Master, such as Active/Good, Non-Stock, Inactive, Discontinued, etc. Whse Stock Flag will show changes to the Stock? field in the Purchasing tab of the Product Master (Default, Yes, No).