Yes, it is possible to upgrade a UC520 16U desktop system to a UC520 24U rackmount system by doing a trade in of the hardware known as the Technology Migration Program (TMP). Unfortunately there is no way to upgrade the desktop system past the given 16Users it was currently designed for. You can find more information about TMP here:
http://www.cisco.com/web/partners/pr11/incentive/tmp/index.html
For user license upgrades on the same UC520 chassis, refer to this page:
https://www.myciscocommunity.com/docs/DOC-1438