Using a Flyweight or Proxy design pattern for an old client and new server

I'm new to design patterns and I was given a scenario involving using either a flyweight or proxy for an airline reservation system. Here the scenario:

An existing airline reservation system needs to be updated. The old system must still be used for a while, even when the new system is being rolled out, because there are too many terminals to update overnight (i.e. the rollout would take place over a few months). Because of this, the new and old systems must work together. Question: what is the best pattern to use in order to allow the new reservation system (server) to work with an old client (terminal)? The choices are Proxy and Flyweight, and explain why one is a better choice than the other.