Are you just using Form Builder to add in validation to the checkout form? If so then you do not need to edit the existing form with Form Builder, instead you can just apply the server side validations to the default checkout page.
This should allow you to add in the validation that you need without having to update any of the form fields and leave the form on the checkout page intact. This is definitely the way that I would do it in the same scenario.
If you are starting with a working set of checkout pages make sure to backup and test each step along the way to ensure that you do not break any of the existing functionality. If you do then you can revert to your backup and let us know what the issue is that you are experiencing.
