Currently on v1.5.5f and OPC v2.0.4 but plan to upgrade to v1.5.6b and OPC v2.1.0. Other major mod updates will include SBA and CEON URI Mapping, Initial investigations look to it being a daunting task.

I've read the install/upgrade readme instructions for OPC. Given the myriad of changes between v2.0.4 and v2.1.0 I wonder whether it would be easiest to first upgrade ZC to v1.5.6b (once released) and then upgrade OPC or vice-versa?