workflow – Re-set Sharepoint 2013 Content Type for all List Records to Default

I had a SP2010 workflow that switched content types from New Items (the default, used on record creation, which only displayed required fields) to Item, which was the content type that displayed all available fields in the Edit form. Well now it looks like Sharepoint admin shut off support for SP2010 workflows and that has caused confusion, with some of my list records using the New Items content type in Edit mode, while others use Item.

How can I re-set all records to a single default content type, to display all available fields? I can change the default content type to Item, which works for new record creation, but doesn’t change the content types used in Edit mode for existing records. When I try to delete the New Items content type I get the error ‘content type still in use’.