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Sorry, but why should OP buy a brand new iMac just to open it? If he/she is a tinkerer, yes, but I personally wouldn‘t mess with the warranty of a brand new all-in-one.
Opening the iMac, contrary to what apple wants you to believe, doesn't void the warranty.
By "some guy" I mean an apple certified technician, thus avoiding warranty voiding.

Finally, the reason you want to do that at all is because a 3rd party nvme drive would be vastly cheaper than what apple offers, and a computer without an nvme SSD is a sad thing.
 
Opening the iMac, contrary to what apple wants you to believe, doesn't void the warranty.
By "some guy" I mean an apple certified technician, thus avoiding warranty voiding.

Finally, the reason you want to do that at all is because a 3rd party nvme drive would be vastly cheaper than what apple offers, and a computer without an nvme SSD is a sad thing.
Well if someone likes that, fine - I would order the computer as I see fit. And aren‘t Apple SSDs quite a bit faster? This is all additional work OP would have to take care of. I, personally, would not go through these hoops.
 
Well if someone likes that, fine - I would order the computer as I see fit. And aren‘t Apple SSDs quite a bit faster? This is all additional work OP would have to take care of. I, personally, would not go through these hoops.
They're fast, but not that fast. They're also way overpriced.

If I were ordering for myself, this is the route I would go
 
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