One address, three services behind it

A shop with three services, each on its own address, and the app knows all three. The catalogue is getting big and you want to split the media uploads out into something else entirely — a different service, a different address. What does that cost?

• Every copy of the app already installed keeps calling the old address, and they stay installed for months.
• Nothing much. The app asks for what it needs; the addresses are a detail.
• Nothing: DNS points the old name at the new service and the app never notices.