Convert plain text to Richtext HTML in Sharepoint online doc library

Currently a document library is not rendering the html content of the multiline text field. The same content type is working perfect in other sites. I have tried to use SharePoint designer and tried to save the column in rich text mode, which has corrected the fields and works when a new doc set is getting created. But the data in the existing library is still showing as plain text, and on edit and save properties of each items it gets corrected to the Richtext HTML mode. My problem is the library has 80,000 items and I am need a way to achieve this conversion of data to HTMl. ALso, the library has lot of folders within folders and document set and then files within. Its nested heavily. Any help is much appreciated. Thank you.