Can I swap the logic board from a 2019 21.5" 4k iMac with a 2017 4k?

@danj I feel like getting a new rear, swapping everything over and then buying a new screen would make it more expensive than finding a 2017 model that’s firmware locked.

I use that example because I’ve seen it in previous models being sold cheap, and I wouldn’t feel bad recycling the board. If the board worked I’d try to sell it separately

@sparklepimp - You would think Apple would have a common case as well as a display which is carried forward within common generations. I have often been surprised that is not the case. So are there any clues when they are? As it turns out there is! Here we need to dive into the Model ID. Let's look at your two systems

iMac18,2 - 21.5" 2017

iMac19,2 - 21.5" 2019

So here we see the major increment of 18 to 19 so Apple see's these these as different . For reference let's look at these two systems

iMac14,3 - 21.5" Late 2013

iMac14,4 - 21.5" Late 2014

Here we can see the minor digit changed and the parts are interchangeable excluding the logic board.

So just looking at the ID Apple see's the '17 & '19 systems as different.

(Cont)

So that doesn't deal with drives or other common interfaced elements like power supplies.

But the case... that's tricky, one has to look at the part numbers to see if they are common (verified as such) so I pointed to above to P/N 923-03079 now let's look at 2017 rear shell P/N 923-01615 TheBookYard crew does a very good job of checking compatibility as you can see in this link both the standard and 4K models share the same shell but if you look at the 2019 link you can see there is not even computability even between the two 4K models. This is a common issue when the boards layout forced the mounting stud to be moved and/or the attached ribbon cable within the shell moved.

OK, let's look at the display and then look at the costs

Here we can see the '17 & '19 share the same display! P/N's 661-12564 & 661-0732 this one is the Grade-A

(Cont)

There is a cheaper version Grade-C at $145 US

So if we look at the parts costs

Shell - $115 ~ 203

Display $145 ~ 261

Somewhere around $260 ~ 464 total cost for the parts

So... is it worth the gamble? It still might be possible to use a 2017 shell you may need to modify it cutting away a stud and a ribbon cable may not route correctly and require the sub part be swapped out from your old shell to make things work. Think of the work having to move over sub pieces Vs the major assemblies.