What seems to happen for me is that Kecleon's type REMAINS Normal
As shown by this replay - I made Kecleon utilise Grassy Terrain, which changes Nature Power's attack to Energy Ball. However, Kecleon's typing REMAINED as Normal type.
I would guess this is because;
The opposing Kecleon used Nature Power!
Nature Power turned into Energy Ball!
Since Nature Power itself is classified as a Normal Move, and chances AFTER Kecleon uses the move (As shown by the text), it takes the first typing which is Normal and not the Grass typing of Energy Ball.