A YouTuber has demonstrated that it is possible to upgrade the older 21.5-inch iMac way beyond Apple's official supported hardware. Unsurprisingly, the upgraded model outperforms the Apple M1-powered 24-inch iMac.
https://www.notebookcheck.net/The-21-5-inch-iMac-unofficially-supports-up-to-Intel-Core-i9-processors-surpasses-the-24-inch-iMac-with-an-Apple-M1-SoC.544036.0.html
Slightly faster, in CPU only, I imagine, and with far worse efficiency...
Unofficially, they support just about anything that can be retro fitted into that chassis. >:D ;D
is there an apple official upgrade service for 21.5 inch imacs owners???
Just because it works now doesn't mean it will in the future. Apple has a tendency of nuking after market replacements, repairs and alterations. Technically hacintoshes work for a little while. Queue an apple update.