If you want to tattoo over an existing tattoo I expect you to some how incorporate the existing tat into the new one, like you would have to in real life! Don't be silly. Be IC. Make the tat artist give you a big black bar over your current tat and then give you a tat on top of that after it heals or something.
And no matter what, do not remove BANISHED tattoos. If you have removed one, xhelp and you will be given it back. Those use special ink that can't be tattooed over.