There are two versions of the 2015 21.5” iMacs the Non-Retina and the Retina 4K display version:
[https://www.thebookyard.com/product.php?cPath=34_165_458_477&products_id=16038|iMac 21.5" LCD Display Panel - Non-Retina - Apple P/N 661-02989 - LG Display: LM215WF3 (SD)(D5)]
[https://www.thebookyard.com/product.php?cPath=34_165_458_478&products_id=15507|iMac 21.5" LCD Display Panel - Retina 4K - Apple P/N 661-02990 - LG Display: LM215UH1 (SD)(A1)]
Now let’s look at the 2017 versions which likewise have two versions!:
[https://www.thebookyard.com/product.php?cPath=34_165_521_522&products_id=17646|iMac 21.5" LCD Display Panel - Non-Retina - Apple P/N 661-07321]
[https://www.thebookyard.com/product.php?cPath=34_165_521_523&products_id=17611|iMac 21.5" LCD Panel Retina 4K - Apple P/N 661-07322 - LG Display: LM215UH (SD)(B1)
From all indications the displays are not compatible either way.