If I’ve already bought a set of plans for my project, can you change them?

No – we must respect the copyright privilege of the owner of those plans. That copyright privilege would preclude creating derivative works – where the original protected plans were modified/ changed. We have not seen a set of purchased stock plans that the owners didn’t want to change; the desire for customization is universal. Once purchased, you should request and pay for changes through the owner of the plan set.