From: Mosquito Bite, Texas
The button below the upgrade button, in most cases, will have text in yellow next to the execute arrow. Click on that text and a list of planes that the unit can "upgrade" to will be displayed. Find some earlier aircraft model, select it, then push "allow upgrades" at the bottom.
When you get back to the main squadron display, push updrade and the older models will appear. As crsutton said, the ones you just replaced will enter the pool of available aircraft for replacement beginning in a day or two. I've found that if there's a lot of planes moved into the replacement pool in this fashion, it may take five or six days for all (or almost all) of them to be available again.
Occasionally, and randomly, problems and solutions collide. The probability of these collisions is inversely related to the number of committees working on the solutions. -- Me.