Upon expiration of the ZCS Network Edition License, the following functions will no longer be available:
Zimbra Connector for Outlook
Zimbra Connector for Blackberry
If you hit your licensed user limit, you will no longer be able to create/delete accounts.
If you do not plan on renewing your license, you can again regain the ability to create/delete accounts by "upgrading" to ZCS FOSS. You should choose the same version of FOSS as you are currently running on Network Edition for this transition, after which you can upgrade to the latest version of ZCS FOSS.