When can you change the bar on ear cartilage piercing?

Do you mean an industrial piercing bar? If so, you can usually go to a shorter bar after a few weeks. If you think that it's ready, go t o your piercer and they'll chnge it if it's ready.

If you just mean a normal barbell in a piercing, switch it out after the swelling has gone down. Only switch it to another steel bar or glass. Don't wear any acrylic or studs in it until it actually healed