Yes, if the trial version is installed and the 21 day trial period elapses, Unity Pro can still be authorized.
There is no need to remove and reinstall Unity Pro.
However, the demo version (60 days) cannot be authorized.
Legacy KB System (APS) Data: RESL208717 V1.0, Originally authored by DaSo on 07/25/2012, Last Edited by DaSo on 07/25/2012
Related ranges: Unity Pro - SoCollaborative software