I was at Apple yesterday discussing a similar question.
I want to install a 1TB drive in my almost out of warranty 24" iMac.
Apple will not do it, not even for a fee, all they can do is REPAIRS.
However your best bet, and one that Apple recommended is to find an independent APPLE AUTHORIZED SERVICE CENTER. Apple cannot even sell you Repair Parts but an APPLE AUTHORIZED SERVICE CENTER can if they choose to do so.
BEFORE you do that, you might want to make sure the Service Center is CLEAN enough to do the job. Do they have a special 'clean room' with a laminar flow hood where they can open it up and do the task? Dust is a biatch when it gets in between the glass and LCD panel.
I would do the drive upgrade myself, but I do not have a 'clean enough' area at the moment.
Apple Tech also warned about handling the glass cover, he said touching the inside of the cover while handling can create a mess of fingerprints.
I told him I have special linen gloves for handling things like that .... my only concern is DUST !!!!
I have replaced the drive in about a dozen 24" AL iMacs, I could do it in about 15 minutes.
eff the whole "dust free" crap with the glass front. In the grand scheme of the whole installation removing the glass is about a 1 on a scale from 1-10. Takes all of 2 seconds. Of course you dont want to touch the inside of the glass, duh?
Just grab it by the edges, theres 3/4" of black you never see.