It is not advised to alter the prefix value on a running installation since it is used as a unique identifier for your specific help desk system. Changing this will cause E-mails for existing tickets to not be recognized and create a new ticket for them.
I changed "TIC" to "IT-TICKET" in the "E-mail prefix" field in the "E-mail settings" section under Configuration > E-mail Settings. The subject in all email now shows "IT-TICKET" as the prefix. Ours is a new install, so I don't know how this will affect existing installs.
There is currently no way of doing this. Such a thing would be possible on the website, but will cause problems for the mailing system. How would you like to display the numbering?