I would highly suggest buying AM2 ears and save yourself a lot of grief however, I love to mod things. When someone once told me I was trying to reinvent the wheel when I created a DIY calf closure clips, I simply replied I'm not trying to reinvent the wheel I just found a better way to mount it on the car.
With that said, costuming is all about fabrication and modification. You're not recasting a complete set of armor here, that shouldn't even be a issue, though there are those that will argue with me for some reason or another.
Bottom line, deciding whether to mod your ears or buy new is solely up to you. Being creative is a part of being human.
On the official side, If you decide to mod just make sure it falls within the CRL guidelines.