Geodata service helps you accessing geodatabase in remote location through local area network/internet. You can create and manage geodatabase replication using geodata service and design for geodatabase replication remains same irrespective of how you access it. Hope this information clarifies your query.
It totally worked. I dont know if its supposed to but it does. We have been holding off upgrading to newer geodatabase versions because of an external host who is staying at 10.1 for the while. I just tested a replication via geodata service between 10.3.1 and 10.1 and it successfully transmitted updates.
You can create replica but our recommendation is to have child replica geodatabase is on higher geodatabase version. The reason being "Geodatabases built using previous versions of ArcGIS do not support some of the newer functions of ArcGIS" Here is the webhelp for more information:
If your child replica is in a later release geodatabase (for instance, your child replica is in a 10 geodatabase, but your parent replica is in a 9.3 geodatabase), be sure you do not add data or make any edits that introduce functionality to the child replica that is not supported in the parent replica's geodatabase.