As long as a ticket has not yet issued, it can always be rebooked or cancelled. For this reason, it is possible with some higher fares to wait until the ticket is issued, while other, lower fares can often require the ticket to be issued immediately.
When booking online with AirManager (booking with registration), you can rebook and cancel your ticket personally, as long as it has not yet been issued.
Once the ticket has been issued some restrictions may apply, depending upon the fare.