Plenty of times I have reset my helpdesk incident number to 1 when implementing or testing. To do this you need to make sure all of the incidents have been deleted. Next in SQL Management Studio you need to open the Altiris_Incidents database and edit the workitem_next_number table. Change the value of next_number to one number before what you want the next incident to be. If you want to reset to 1 then this number needs to be zero.
If you had a slew of incidents created on accident from an auto reply to your helpdesk then you may need to change it to maybe 12860. 12860 is your current last incident and the next new incident will be 12860.
I have used this many times and have had no real issues that I can find.