Make sure they understand that they were previously receiving the hugely discounted introductory rate, that you love working with them, and give them your "norm

Make sure they understand that they were previously receiving the hugely discounted introductory rate, that you love working with them, and give them your "normal" rate, which is whatever price structure you think will work better going forward. They may try to negotiate your new rate. Give yourself a little wiggle room so that you can accommodate a negotiation if necessary. If they flat out refuse, then take it as a learning experience and spend your time finding a new client.