Our dept reorganized and now there is a new person who is handling my customers.

I ran into this when I worked as an assistant to a financial advisor. Some of clients who had smaller investment portfolios were given to a newer financial advisor. This was a decision that was made by the company and I had nothing to say about it. Being that I was the point of contact for many of these clients, they still continued to call me and were not happy to learn they were assigned to a new advisor without having any input and some even demanded to be reassigned back to their old advisor. When I was honest with them and told them that old advisor had transitioned to only working with clients that had a certain size portfolio and talked up the new advisor by saying "He was great and will do a good job for you," this made them happy. In general, people just don't like change but can learn to deal if they have to.

The simple answer is nothing. Running interference will never end well. If you get contacted by email from a customer, reply that you will pass the information on to their new representative with copies of the reply to the new person and their/your boss(es). If they call you, email and copy the same people (but not the customer). It is out of your hands now, and I know that you care about your former customers, but for some things, you have to let go. The new person will either get their act together, or your company will make changes to fix it. The customers may leave for a competitor, but you don't want to be in the middle of that fall-out (trust me on this one, been there, done that, have the scars).