If I took it to Joe Smith who makes aquariums and said "Can you please remove this internal overflow and equip an external overflow on the back?" How much would that cost?
I looked at the site and I don't know if I trust myself to cut glass
No idea on what a store would charge as that is an "a-typical" job. Contact Max @ The Aquarium Showroom in San Jose, he'll quote you over the phone most likely.