Recently I had a chance to Book my Tatkal Ticket in IRCTC through Online. There is no ticket available for the Source Destination of my choice. Fortunately the ticket is available from another destination which is around 3 stations prior to my place. So I booked the ticket and later wondering what would happen if the Ticket Examiner allotted my seat to somebody. Then I could find that it is possible to change the Boarding Point for booked tickets also in the IRCTC Website.
In the Login page, We can see the Booked Ticket History, double click on that it will take to the booking history page.
We have the option to choose the Boarding Station while booking the ticket.