Is the old hostname going away entirely? Or will it continue to be served by mail-in-a-box, and forward mail to the new hostname?
The only good way to do this would be to have a flag day... On a day that you choose, the old hostname goes dark and everybody must update their client configs to continue to use the system. (you'd send an email describing how to do this of course)
With some effort, and if you don't mind doing wildly custom and unsupported configs to mail-in-a-box, you might be able to keep both hostnames alive so people can continue to use their old client configs. Their mail would still have to be sent and received with the new hostname.
And, if the old hostname is going away then no, it's impossible to push new configs to clients.
(if it were possible, I'd like to know how! client config is easily my biggest source of problems with mail-in-a-box)