It was done via hex edit presumably. I haven't put this in the Hex Edit Repository because it hasn't been fully tested yet, but you're welcome to try I guess:
Codeorg $0781B8
db $25,$00 ; change what the Egg Plant shoots (sprite F4 on X=0)
org $0782F9
db $63,$01 ; change what the Needlenose Plant shoots (sprite F4 on X=1)
EDIT: these values should also be edited:
Codeorg $078250 ; change what the Egg Plant shoots (sprite F4 on X=0)
db $25,$00 ; when you do a ground pound near them.
org $078347 ; change what the Needlenose Plant shoots (sprite F4 on X=1)
db $63,$01 ; when you do a ground pound near them.
Note that, the second byte is only used if the sprite ID is over 100, so it becomes $01, being the first byte the rest of the sprite ID (that's why, in this case, the Needlenose Plant shoots the sprite 163, which is the Bouncing green Needlenose) You can check sprites' ID via Golden Egg or in
this page. It's more than obvious to say this changes are global and that NOT every sprite will work.
(Just as a trivia, you can make Egg Plants shoots themselves LOL)