DNIS is kept afaik. In order to reroute to a new mailbox I sometimes use a dummy ACD path which interflows to VM where the extension is the ACD path's number. I guess there are more roads that lead to Rome in this case, so there might be better solutions. The latter usually for me depends on, if I have more to work with then just a Mitel 3300 platform, eg IVR etc.
Cheers,
Dutch