You did not mention what version of exchange you are using. Please provide those details. Thanks.
That will help me know what tools you need to use.
First, for this user, did you uncheck the box “automatically update email addresses based on recipient policy”? if not, the old policy will be re-applied at the next update and put the old address back.
Also, did you update the recipient policy for all users to reflect the new format you want to use? If not, they will all get the old policy when the RUS runs.