ON 6.2.2 snapshot - If one tries to use an m5 style form which includes 
info.magnolia.contenttags.form.field.TagFieldDefinition in an M6 app, like the pages app, then the form will display the message: "Conversion not supported."
Log shjows:
"ERROR inition.migration.FallbackFieldDefinitionConverter: No converter for old class info.magnolia.contenttags.form.field.TagFieldDefinition. Falling back to class info.magnolia.ui.field.StaticFieldViewDefinition."

The Content Tags field is a commonly used one and should be converted.