Its not uncommon that kanji could be further simplified by using radicals taught later down the road in wk. While in this case, it may make the mnemonic easier to wrap your head around, I don’t see it as a reason to shift 化 up two levels.
I just don’t think the work needed would make enough of an improvement to justify it. Of course, if its much easier to remember it as change + grass, then you are more than welcome to use that. Like I said, there will be other times will occur and on top of that, there will be times where you simply don’t like the mnemonic. Making mnemonics that work well for you is a good habit to get into.