Animate a rectangle without modifying its rounding. How?

You could also achieve this by setting the desired Roundness value (Contents > Rectangle > Rectagle Path> Roundess) and then adding an Expression to it (Animation > Add Expression) as well as adding an Expression to the Scale (Contents > Rectangle > Transform: Rectangle > Scale) while at the default 100, 100%.  Divide the Roundess Expression by 1 and mutiply the Scale Expression by 1.  Then link the "1" value of each Expression to the same Slider Control (set the Slider value to 1.00 before linking to avoid having your Rectangle disappear).  For however much the Slider Control increases or decreases,  the Roundess and the Scale will change in proportion.

Although, this will not maintain the Stroke thickness.  You would need to promote the Stroke (Contents > Rectangle > Stroke) to the root level of the Contents and arrange it behind (or below) the Rectangle.

I'm stacking multiple Shape Layers for demonstration.  

Create one with "Rounded Rectangle Tool" then in the Layer options modify its size of "Rectangle Path 1" but first deactivating the chain of "Constrain Proportions" to unlink the X axis from the Y axis. This way you will not damage the rounded corners while you modify its size in width or height.