No, they cannot "change it to an installment loan."

In fact, you have no type of account agreement with a debt collector.  Thus, you dont have any billing statement due dates upon which to base any account delinquency.

If you see reference to "installment" in connection with a collection, it is a reference to the type of credit with the OC that led to the collection.

Collection activity must all cease to be included in your CR upon expiration of the 7/180 period from DOFD.