I don't know if you can get the old one back, but I strongly recommend making a custom form for your form and setting that as your default. If you alter the default system form, you run the risk of MS overwriting those changes in an update.
If you make a custom one, even if they change the default back to the system form, all you have to do is change the defualt back to your custom form, no muss, no fuss. (also, if you install any managed solutions, they won't mess with your custom form)
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