There are a lot of different models for software testing, and you have to find one you like, and then implement it.
In a very general sense, you want to be sure to include the following elements in your test plan.
- Out of the box functionality for basic use cases (Search, Create/Update Part, etc.).
- Workflows of any kind
- Custom code of any kind
- Access control based on what you expect to happen based on your access control plan. Once you have validated your access control plan, then you don't have to retest this on upgrades since upgrades won't mess this up like it often will with workflows and customizations.
- Performance (click to result) for any slow or critical tasks
- Load testing (use an automated tool to simulate a large concurrent load to see how your system performs under high concurrent use)
- Process testing that validates your business process as it flows in and out of Windchill as a supporting application.
Al