Here's a mini-tutorial covering
After Effects' rounded rectangle tool and how you can alter the radius while drawing.
It's easy to change until you let go of the left mouse button, but it's common knowledge that you can't alter the corner radius later. Welcome to a special Myth Busting episode of Just the Tip where I bust that belief by solving the problem. I also talk about using Shape Layers instead of masks - as they do have a roundness option.