Is it possible to upgrade the motherboard/CPU/RAM?

Sorry Apple sometimes makes orphan systems ones which are the only one in a series. Intel is the root issue here as they brought out a few odd ball CPU’s as they needed something to fill the void of not offering the 10 nm chips they were expected to deliver!

So here is the different series Apple intro’d around that time:

• 21.5” 1.4 GHz iMac Core i5 (Mid-2014) I5-4260U (Haswell) - iMac14,4
• 21.5” 1.6 GHz iMac Core i5 (Mid-2015) I5-5250U (Broadwell) - iMac16,1
• 21.5” 2.8 GHz iMac Core i5 (Late-2015) I5-5575R (Broadwell) - iMac16,2

Note the Family ID (last value in the description) is different between each. You can’t swap out the CPU’s as each one uses a different micro code (Haswell Vs Broadwell) which is held in the EFI (that’s typical for Apple). Replacing the logic board would be the only option but again Apple makes it hard! There are differences with the display connections between the (iMac16,1 Vs iMac16,2)

Here’s an example where Apple used the same Family ID across very different logic boards iMac16,2 Here Apple used the same CPU series across multiple systems (same micro code) but different logic boards & displays!