The eBay article that you linked states that:
With the latest 27' iMac models offered since early 2013, the release latch for the stand assembly that was available on previous 27" models has been removed.
The 21.5-inch had a model added in March 2013, which may have also allowed for the VESA model, which is also at the time that Apple started selling the special VESA mount models.
It may be that Apple made a production change in all iMacs manufactured after that time, to allow for the VESA mounts. And, (just a guess) that the company that makes the adapter isn't just guessing.
(Maybe it also means that, for a few months, the Late 2012 iMac 27-inch DID have the release to remove the stand from the outside. You would have to determine when yours was manufactured, and know where yours is in the production run. There's probably no way to find out when some minor change like that was actually included, but sometime in early 2013, I guess.)
Anyway, sounds like a messy proposition to change out the mount.
I suspect that even if you have an Apple service shop do it, they will mention that Apple will no longer keep it in warranty. It's just too major a mod (that's just my opinion, but ask Apple about it, too. I'm not Apple, obviously!)
Probably 2 or 3 hours labor (maybe more) to install a $50 piece, won't be cheap..