Could I exchange a new MacBook Air for a 27" iMac

You have to ask Apple. We are end users like you. Youre return/exchange options differ depending on from whom you bought your Mac. Please see;

Returns & Refunds - Shopping Help - Apple

FYI, they do not do "exchanges"; as mentioned, you have a 14 day return period, so you can return this for a credit and then buy something else.

If you bought it directly from Apple, the return period is 14 days.