Honestly, the easiest way for me is to just use ExGFX. Open GFX01 (in which the Piranha Plant is located) in YY-CHR and replace the fish at the top of the screen with a stem. Then, save this as ExGFX**.bin. Just use this as a substitute for GFX01 in SP2 anytime you want normal Piranha Plants - that way, you can still have floppy fish elsewhere in your game (just not in the same level as Piranha Plants).
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If I moderated your hack, there was apparently a 90 percent chance it was rejected.