hmm...
The term "degraded" is pretty important. If you've really done nothing with your PC as far as software changes or Microsoft updates, and the LCD has truly 'degraded' then by all means swap it out.
IF however, you've arrived at a rather hasty decision that the monitor has degraded, I would challenge you to think at least twice. I say this because there are millions of little LCD pixels in this unit. If you've got a good one and there are no/few dead pixels AND your light leakage is good, then you should really go to many lengths to be absolutely certain that the problem is with your LCD. If you go out of your 21 days, you'll have a far worse time dealing with Dell. Remember that shipping time is INCLUDED as dell goes by INVOICE date not "received date." So let's play worst case scenario - let's pretend that there is a complication - lets say that the problem is your video card drivers, and you return a perfect monitor for one with 2 dead pixels, and you are now 45 days into your purchase, you are in deep deep trouble and will spend many hours hassling with Dell.
I offer this because in several years of heavy investigation of display devices AND video cars AND PCs, I've not seen a single claim of a person's display "degrading" over 2 days. Plus, your wording "a known good one" scares me. Sounds like a refurb.
My advice is to take some time out of your life, hook the monitor up to a different PC, experiment, do video driver upgrades, video card swapouts, try a DVD player, and be very very cautious about drawing the wrong conclusion. However, do your testing QUICKLY and THOROUGHLY becuase you want to get a 2nd chance within your 21-day window. One of the drawbacks in dealing with Dell is that within your 21 days they will do anything, but outside of the 21 days you will go through HELL getting a refund, getting a replacement especially if you swapped out a good one with a bad one....
One thing to consider as your continue down yoru journey is to return the monitor for a full refund. Then you can repurchase, and begin the 21 day journey again...
I have learned much from dealing with Dell across 2 LCDs and 3 laptops and offer this advice with several battle scars!!!!!
