If you watch Luke’s video he gives you the blow by blow conversion of a 2011 27” model.
While this is a fun project since Apple is presently not offering a 27” replacement after intro’ing the new Studio system the other day. I’m not sure if you can modify a 21.5” system and the 2012 series is even thinner than the older 2011 and older models.
If your 21.5” is still working I would hold off doing this. I would sell it and just buy a 24” model vs buying a M1 mini to then convert it.