Joe Thomas
Hi Joe,
If you have an existing cache, then the steps you will want to take to have a new service utilize the cache is to publish a service with the same name as the cache folder in your cache directory on the server machine. You will then want to ensure that the service properties are set to draw the map service Using tiles from a cache. This setting is found in the Caching tab in the Service properties. If all is set correctly, then the new service will try to look in the cache directory for a cache with a matching name and if it finds one, it will use that cache.
There are some other nuances that should be investigated if you are using custom tiling schemes or the like, but this is what you will want to look at first.
I hope this helps.
-Daniel