Chosen Solution
I’m using older iMac 21'5 late 2012, can I replace the screen with the newer retina LCD? Please advise,
No, unfortunately not. The main issue will be the logic board, which is only capable of outputting HD (1920 x 1080). so even if you could retrofit the display, there would be no way of getting the GPU to output to 4k resolutions. But aside from that, the imbedded DisplayPort cable, the LCD port and the logic board port are all different between the models. The only similarity is the dimensional size of the machine unfortunately.
Sorry no the older 21.5" models use a different interface logic than the newer 4k models.