If you already have activated VisualCron on one server you must de-activate your old install first before being able to activate on a new server.
To deactivate your old license you need to:
1.Install the trial on new server
2.Connect to the Server with the Client
3.Export any settings from old server through File menu->Export to a secure location
4.Import any settings in new server through File menu->Import
5.Choose Deactivate license in Server tab->License->Deactivate
6.Activate your license on the new server
Please note that Deactivation / Move a license requires that you have an active Maintenance.