Replace or return?

There's no issue with that. You can just return it and order the new one and refresh the 14 day return period. Applecare is also refunded.

EDIT: Do check Apple's refund policy for BTO machines in regards to your country as it does seem to vary.

 

No issue with that - I did the same thing about a month ago. I had bought an iMac, and it had fibers or something under the screen. I took it to an Apple store to have the screen cleaned under warranty, and instead of doing anything, the Genius Bar guy just put a great big scratch on the aluminum facing, as well as a dent in the bottom left edge when he DROPPED THE IMAC.

I called Apple about it, and they were super helpful. The woman who helped me had a new iMac out to me that day with overnight shipping, and said I'd get my refund as soon as they received the damaged one back.

The entire process took about a week, and was worth the hassle to get a flawless iMac.