Just for the sake of discussion, and to play devil's advocate, let's take a minute and look at the practical effect of this action.
Other than the fact that OP is returning a different unit that he purchased, what is the harm suffered by the big box retailer? I'm sure there are some in terms of opportunity cost, restocking, labor, inventory control, etc. But the retailer gets an unused, unopened, product that they can (I'm assuming) put back on the shelf. At worse, it goes into the "returns" pile and is dealt with along with opened returns, right?
Now, compare that to a user purchasing a product from the same retailer, using it, and returning it. Totally within the return policy, but the user sends back an opened, used product that definitely cannot be restocked and sold as new. Retailer reports such as the number of big screen TV sales/returns immediately before/after the Superbowl surely suggests this happens intentionally and with some frequency. And it's probably not just $2000 big ticket items. Some people fully intend to return, some go in with the default position to return unless they are blown away with the product, etc. Is this behavior viewed with the same revulsion? Based on net effect, I would think it should be even more frowned upon. But shops with generous return policies encourage "try before you buy" and some even bank on people who want to return but never get around to doing so.
All this to say that interestingly, the "harm" here seems to be fairly low. The end result is that the user gets what he originally wanted from the dive shop, and the big box retailer gets a new, functional, unopened unit back. A lot of people regularly do worse, with worse intentions, and we don't bat an eyelash at it. Not to say this makes it right, but it is interesting for us all to consider how our own moral compass is calibrated...
it seems to me that you are arguing that since other people do things that are more immoral, then this action must be moral.