When you upgrade your ECC or ERP system you will also have to plan activities in related systems. For instance, you will probably have to check BI data extraction in order to check everything is working fine, PI , SRM, APO, SCM integration….
A simple example. In my last upgrade we have a problem with SRM syncronization. Middleware in SRM needs to be reconfigured. There is a lot of documentation on how to do this, including several changes in middleware. Once applied, we still have problems. “The table CRMATAB in the ERP system is empty, Run report SMOF_FILL_CRMATAB”. However our SRM system doesn’t have this report. Solution. Implement Note 1501685 – Table CRMATAB is empty. in SRM!!!
So, keep in mind in your upgrade that you will probably have to do a set of things in related systems. This could affect your upgrade landscape. For instance, if you plan to use a sandbox system to test the upgrade, it will be a good practice to also plan a sandbox of related systems in order to be able to test integration scenarios, apply corrections, etc., without impact your maintenance landscape. We do it and we success.