You don't need to delete a machine when you reformat your hard drive or update your system. Our servers will still be able to recognize that you’re using the same computer, so you can reauthorize it without wasting an authorization.
Also, keep in mind that if the same machine is deactivated more than once, it will not be counted again. But if you do frequently need to move your authorizations to unique computers, please consider keeping at least one of your three authorizations on a flash drive. Then you can freely move your licenses from one computer to another without worry of running out of deletions again. Here's how to do that:
Make sure your usb drive is in your computer. Then, open one of your plugins in ProTools (or any host) and click the key icon on the bottom right side of the plugin window. An authorization window will pop up. Under the text "Select the device you would like to activate" there is a pulldown menu. Click it and select your USB drive. Make sure the "Activate all my licenses" checkbox is checked. Then click "I'm Online" and enter your login info. Now your USB drive is authorized and can be used on any computer.
One more thing, in case you didn't know: There's almost never any need to individually deactivate your plugins one at a time. Normally, you can just go to Manage Devices and there you'll see a list of your authorized machines, and a "Deactivate" button that removes all the authorizations from a machine all at once. It will make your life a lot easier if you frequently upgrade systems.