Oh. In that case I don't know if I'd be able to help you with that. If you wanted to switch the fanfare that plays in all instances, all you'd need to do is either change the pointer to the header or change the song that plays. The former is a simple change, and the latter would require hunting out the place in the ROM that controls what fanfare is played (although it'd technically be the right way to change that).
However, I can't say that I'd be able to make it so that the item fanfare plays when leveled up in battle and the level up fanfare plays when leveled up with a Rare Candy. There's a chance that both are dependent on the same code. In that case, a rewritten ASM routine would be needed, and I wouldn't be able to help with that.