Is your Dynamics 365 Field Service solution upgrade from 8.x failing? You might want to try this..
Not long ago I raised a support case because I was having an issue upgrading a Field Service environment from version 8.2 of the solution to 8.3 and I duly got a simple resolution from the Microsoft Support Team which worked perfectly. Then today i ran the upgrade from 8.3 to 8.4 and again it failed, but the resolution I originally had from my previous issue also worked with this one so I thought there may be other people in the same boat who may benefit from the very simple suggested fix although obviously this may not work for everyone. So here it is with no guarantee's it will resolve your upgrade issue!
You need to go into the default solution, and locate the 1:N relationships under the Case entity. Find the "msdyn_incident_msdyn_workorder_ServiceRequest" and double-click to edit.
Click on Mappings.
Then delete the "customer id" mapping from the top row as highlighted below.
After this, save and publish the customisations and then try to run the solution upgrade again. With any luck it will go through successfully this time and the mapping should automatically get re-created after import.