When you are planning for your tests, either small-scale plans, just to look at a single feature or change, or more complex plans for testing and application there are a few steps you can go through to help ensure you have to revisit your plan only when your understanding of the SIT changes rather than the requirements for the plan changing.
Be clear from the start who is using your plan, is it just for you to help plan what you are going to test? is it for the test team to allow for a shared understanding? or will it be used by the wider project?
Having an understand of this will enable you to include the information that the rest of the audience need for the plan to be helpful for them. You may find that a single plan isn’t sufficient as different people require dramatically different information.
Where the test team may only need a loose plan to help the achieve good coverage the project manager could be wanting to understand resources or duration and stakeholders may want to be able to see that what they are worried about is getting sufficient testing to give them comfort once live it will be reliable.
Understanding the differing requirements for any plan you make can help you produce one that is fit for purpose, in my experience the easiest way to do this, is to create an outline of the plan, something that doesn’t take too much time to do, and share it with those who will be looking at your final plan, speak with them about what it tells them and what they need to know that they can not see, incorporate what you can, make compromises where there are too many conflicting needs.
This will help you create a plan that works well for most people the first time and hopefully reduce the number of times you need to revisit the plan.