I wonder if it is possible to change the displayField of a feature class that is returned from a Feature Service in ArcGIS Server Enterprise
I am using ArcMap v10.6.1
This is handled within ArcMap, specifically within the layer properties. You can change the default display field for any of your layers, re-save the MXD, and then re-publish your service to get the effect you're wanting.
Retrieving data ...