John,

I agree with Wayne. Make the server fault tolerant and forget about changing the app. With each adjustment you make in the app you create new problems.

1. Register 1, 2, and 3 all get their data from the server.
2. Register 1 sells two boxes of cereal
3. Register 2 sells 1 box of cereal
4. Register 4 comes on line, how many boxes of cereal are there?
5. The manager changes the price of cereal at the server, how long before the registers know this?

Don't change your app, make it work against the server always.