How to add a new field in Address details pop up in sales order

Dear Experts,

I have a requirement to update the Address details in Sales order page.

In sales order page ,on click of Address Details link a popup opens to update the address of the customer account.In this page ,currently based on the Country selection State gets populated. We display state in the format of State-Region (Refer screenshot AddressPage1)

Same case on the Additional Fields popup also (Refer screenshot AddressPage2)

Now below is the new change from business.

1. Remove the Region from the State dropdown .
2. Add new dropdown with list of regions below State dropdown .The regions should be populated based on State dropdown selection.
3. Add a new text field next to Region
4. Do these splitting of State only for specific countries .

Please let me how can I achieve these .

Which is the business object and screens where I can edit.

Any help is highly appreciable.

Thanks,

Resmi