Stroppy,
Go to your Sent Items folder and find the appointment. Right-click the appointment and click Resend. Make the appropriate changes to the appointment (add/remove users, change date/time, etc) and click Send. It will ask you if you want to retract the original; click Yes.
This should do what you want.
-Ron
P.S. Alternately, you could create a new appointment and ONLY include the new person (be sure to delete yourself from the TO field).
We all play from the same deck of cards, it's how we play the hand we are dealt which makes us who we are. -Me
murof siht edisni kcuts m'I - PLEH