I would look at this two ways.
Generally speaking, once you make changes to a system as far as OS, you may or may not have voided warranty (return) at that point.
In every case, when you return a computer they are going to wipe and reformat it anyway, so they may not care. That machine will not be sold as new, will be marked open box, or even sold to an "auction type" retailer, etc for resale.
If you were returning it for a like model they probably wouldn't care and just exchange it, especially big box stores...but in the case that you want to refund exchange you may have boned yourself when you changed anything.
I would take it back and hope for the best. See what they say/do.