[ASM & Hex] Changing OW sprite in-game

So what I wanted to do in my ROM is using a flashback (and that means changing the hero sprite). Anyway I've tried an ASM routine FBI made (https://www.pokecommunity.com/showpost.php?p=8620314&postcount=465) but he told me it has some bugs and I've realised too myself, (new sprite disappears after I choose any option from menu is an example).

Anyway, is there any routine for this or any other way to do this? (Except JPAN's engine btw)

Hey did you ever figure out what to do?

The CFRU has a couple of vars that may help you just do that