I always ended up just creating a new rounded rectangle if the existing one don't work. Is there a way to modify the corner radius without having to create a new shape?
I know I can modify the bezier handle manually one-by-one, but I can't figure out how to do this in a more accurate way for all 4 corners.
It seems like the best way is to use the Stylize/Round Corners effect on a regular rectangle. This way the corner radius is changeable and independent of the rectangle transform property.
With this method, the Appearance panel will have a Round Corners setting for any round rectangle paths. This will allow you to edit the corner radius for existing shapes.
In the new Adobe Illustrator CC, you can use 2 easy methods to change the corner radius of a rounded corner rectangle that has already been drawn:
Simply hold and drag the round blue handles in each corner.
You can change the border radius of all four corners with precision by using the Shape Options and simply enter a new measurement for your corners.
To change only one corner or give a different corner radius value to each of them, click the chain in the middle and fill the corner radius field for each corner.
While drawing rounded rectangle (while still holding your mouse button), press UP and DOWN arrows to change radius. This is a common way of changing attribute values of certain tools in AI.
Try using Convert to Shape effect: Effect → Convert to Shape → Rounded Rectangle.
Seems you've already figured out that the Stylize Effect is a better method. If you want to go one step further check out VectorScribe from http://www.astutegraphics.com it's a fabulous plug in that will allow you to not only continually round any corner but also had many other very useful features.
In case anyone comes across this question in 2021 and is tearing their hair out, here is an updated solution.
Illustrator allows you to edit corners precisely in the Transform panel only when the object is a "Rectangle Shape", which therefore has "Rectangle Properties", which include corner radius.
Drawing a rectangle with the Rectangle tool creates a "Rectangle Shape" object.
Any other way of creating a rectangle (e.g. if you load a file from an SVG perhaps) will give you a path that happens to be rectangular. Illustrator won't recognise it as a "Rectangle Shape" object.
To convert a rectangular path into a Rectangle Shape:
Select the rectangles you want to convert.
Go to the menu Object > Shape > Convert to Shape
This will create a Rectangle Shape of exactly the same size.
Note that if the path is even slightly not rectangular, it will create a "Polygon Shape", which won't work. If this happens, you will have to go through the points and line them up first.
Now you can select all your Rectangle Shapes and edit their corners in the Transform panel.
