I do lot of video editing work, mostly on FCP & Avid and I feel Quadro card is way to go. Do you think I can get it changed from Apple?
I don't even think the hardware would be compatible. Quite frankly, while you can open up the system and tinker with it, you have to worry about two things:
The iMac is a turnkey system, not an upgradable machine, like the Mac Pro (or, gasp, a PC). I'm sorry that this is not the answer you want to hear, but I don't think that Apple will do it. That said, it can't hurt to ask.