The testers need to know two aspects of change:
1. The characteristics of the change so that modification to can be made to assure the right functionality and structure
2. The version in which that change will be implemented.
Then follows with Risk Analysis which helps for further testing
Treacebility matrix is best to trace the changes in current functionality
Pick one that embraces constant change, but if requirements are continually changing you need one that matches the environment so you need more than a testing approach, you need a full process approach. Agile is one that embraces constant change.
In this situation you need a broader scope. Otherwise you need to be extremely flexible in any test process you take as everything is changing underneath you.
Nothing learns better than experience.
"So as I struggle with this issue I am confronted with the reality that noting is perfect."