changing your order questions

Hey Guys,
I ordered the 24" iMac and decided after placing the order to upgrade the hard drive. Its been 3 days now since I asked to change the spec and the apple reps dont seem to have an understanding of the policy surrounding this. I phoned this morning and the guy said order management wouldnt allow it. Then five minutes later another rep emailed saying order management were happy to upgrade the system if i sent a confirmation email. I am not confident the order will be changed can anyone reassure me? Has anyone changed their order after placing it. I dont really want to cancel and reorder as ive already waited 7 days for it to be built.