In order to prevent inadvertent edits to an ID field during data maintenance with ArcGIS Collector, I set the field to "read-only" in ArcMap prior to publishing the feature class to an AGOL hosted feature service.
I exported the feature service back to FGDB format at the conclusion of data collection, but could not un-check the field's read-only setting. See the attached example.
I can open an edit session on the FGDB, I can add fields, I can delete fields, and I can do any other sort of edit, but I cannot set that field back to editable.
The exported FGDB is not a replica, It appears to be a FGDB of the plain vanilla variety.
The work-around was easy enough...just add a new field, calc the ID values into it, and delete the original field. No problem.
However, I remain curious about why I was prevented from setting the ID field back to an editable state. Any explanations are welcome.