In order to achieve this we need to set the TCL API path in the Spirent TestCenter GUI session profile properties with the required version path of Spirent TestCenter installation directory (where pkgIndex.tcl file is located), as shown below
Note: The environmental variable "TCLLIBPATH" should not contain any Spirent TestCenter installation path otherwise, iTest Spirent TestCenter gui session will always connect to version specified in the env variable.