I have found that just changing the $column.type doesn't quite do enough when changing from a single line to a note type. Some of the parameters still looked like they were coming from a text field instead of a note field. The trick turned out to be also entering the new $column.SchemaXml information, then the full field updated the type correctly

@Michael, how did you go about changing the schemaxml? Did you extract it and update some tags then just $column = $modifiedXml? If so then what tags where required? Thanks for a great post!