We are upgrading a large enterprise GIS from 10.1 to 10.5. Ideally we would like to import some existing tile caches but it looks like this is not possible?
You can copy the cache to your new cache directory and publish a service with the same exact name. The data frame name should also be the same. Set the service to render based on the cache and it should draw from the cache automatically.
You might want to keep an eye on the status.gdb for this cache. After you copy the cache, the status.gdb will be at 0% because you copied the tiles (bundles). You would then want to run the Manage Map Server Cache Status tool so that the status.gdb reflects that the cache is at 100%. This functionality has not worked properly for me in the past when copying caches so I cannot guarantee it will work. If the status.gdb is not that important to you, then using this tool or the fact that it does not show 100% is not a big deal.
The data within the feature classes in the status.gdb determine the completion percentage of the cache and that shouldn't be erased by simply moving the status.gdb to a different directory. It may be a good idea to run the Manage Map Server Cache Status tool regardless if the cache status is 100% or not, as it'll help tell you if the service sees the cache is there.
In the past whenever I copied a cache, the status.gdb reported completion percentage to be 0% even though I copied a complete cache from 1 server location to another.