You can't reschedule the meeting without the organizer (the person who originally created the meeting). You'll either have to A) hop onto his account, b) bother an admin or C) create a new meeting.
C is the preferred route. Then just get everyone to accept the invite.
edit: I've done this in Exchange 2010 but not Exchange 2003
The procedure is much the same.
You'll need to grant yourself both Full access rights and Send as rights(I know, Full Access should give you all right, but it doesn't. I'm sure there is a long history lesson about that), because after you modify the meeting, you'll need to resend an invite to everyone. There are more details, but I'm not sure exactly how to do it in Exchange 2003. Here is the technet article.
BTW, any mailbox that you have Receive as permissions, also shows up on Outlook, so be prepared to download all their emails. Or just use OWA.