Hello,
It is a best practice to test such modifications in a staging environment before attempting anything direclty in production.
When you extend an existing table with new fields, it is not supposed to change the original schema but to add a new schema with the same name with a different namespace (for example an extension to nms:recipient would be stored in a cus:recipient or kcm:recipient as in your screenshot).
But if you modify a schema I think there is no way to revert what you've done as it updates the database structure.
Have you checked the configuration guilde of the documentation?
Also, you can find an exemple of extension here.
Hope this helps,
Florent