Maybe the pawn was at one point partnered up with a player that pulled off the Lasker trap, where underpromoting to a knight comes with check and leads to a win (althought that's an opening by Black, and here she's clearly White).
And ever since, she's been wanting to turn to a knight again, not knowing it's something done only very rarely.
Neither the image nor the tags on the pixiv version's tags specify if this "promotion" included a physical transformation. For reference, the artist's other works mostly focus on feminization with the "shemale" tag.
