Cool idea!
If this were to happen, it gives an unfair advantage. Things from the store are to be completely cosmetic. If you can "fly" a block or two higher when transformed it means you can get to places others cannot.
For example:
You're transformed into your owl, and you join a quest/event world. Assuming you can actually join the world disguised and that you remain transformed whilst in that world, even if magic is off you will be able to get to certain areas you may not supposed to be able to get to. Even if they fix this (if this is even accepted!) there will be a way to abuse this, there always will be - it's just a matter of if somebody finds it or not.
I might just be misunderstanding, but overall I don't think this idea would work nor do I think it'd be accepted. Still a nice idea though.