The reason for separating them is performance. The QlikView server is monitoring RAM usage and when too much is being used starts to close down documents in the cache. With way to much RAM being used the server can get problems. A Publisher will at times also consume large amounts of memory and this makes it difficult for the server to "plan".
If it is working for you there is no reason to move them to another server. It all depends on how your applications are using resources and what server you have.
If you decide to move I guess that it is easiest to move the AccessPoint to another server (depends on if you have 1 task and 1000 cals or 1000 tasks and 1 cals). In this case you install QlikView server on the other machine and make the setup in the QMC on the Publisher-machine. Read the manual about which services to install on the AccessPoint and how to configure. You probably need to go through the tasks in QMC to make sure that distributions go to the new AccessPoint. CAL assignment will probably also be lost.
You can shutdown unnecessary services (QVS and QWS?) on the Publisher machine afterwards.
When it comes to licenses it should work unless you try to run both the old server and the new a the same time.