System dependencies
See which processes and activities rely on particular applications.
Talk to the people who use it, follow the work around it and uncover the manual steps, dependencies and workarounds the new system will need to support.
A frustrating system does not always need replacing.
A frustrating system can be part of the problem without being the whole problem. Follow the work around it before deciding what the replacement needs to fix.

See which processes and activities rely on particular applications.
Find where people compensate for technology.
Identify where people enter the same information multiple times.
Understand where different teams depend on the same application.
See what happens before and after a system-supported activity.
Understand what replacing a system could affect elsewhere.
Start with the work that depends on it.