Hi there,
I've had similar issues:
I recently upgraded to 10.3.1. I am using GeoEvent Processor with Portal for ArcGIS. My requirement is to update incidents in a hosted feature service. The attribute names of a hosted feature service are all converted to lowercase names, such as alerttype, because of Data Store using PostgreSQL DB. The GeoEvent definition incident uses alertType instead. So I copied the incident definition to a custom definition and altered the attributes name to match with those in the hosted feature service.
So, based on your advice RJ Sunderman I tried to use a Field Mapper processor to convert these names and it worked perfectly. Here's the according GeoEvent Service (it's in German though).
Thanks for this great question and advice,
Philip & Marko Einspieler