Long time running production services and maintenance

we have many old services running in production for months or years with no new functional requirements. let's say there are 100 such services and now one oracle upgrade comes which while doing analys

we have many old services running in production for months or years with no new functional requirements. let's say there are 100 such services and now one oracle upgrade comes which while doing analysis we found will impact 20 services and we have done the changes for them but in this we might have missed some services that was using Oracle and we were not able to find it in the analysis. There are some way to tackle this- 1. regression test suite - we run it post any of these kind of upgrade comes but they are slow and brittle. 2. tagging approach- we create tags based on services oracle in this case and put it on test cases. we will now only run regression test cases that have impacted service tag. tag management is a con. what other approach or strategy we can adopt to maintain our production services.