When a user logs into the "MyDevices" web site and then installs the exchange profile it knows the user to use for those settings. You can change the assigned user in ProfileManager by selecting Users, then search for the user and add the device to their name. I don't think that will change how the previously installed profile behaves though. Delete the old profile and have the new user log in and install it for them.
I just spent a while debugging this myself. Internally, Profile Manager caches the configuration profiles that it sends out per-device (or something like that). It does not reset the cached profile data when you change device users, so you keep getting the older version of the payload. The only way I've found to address this is to go mucking around in the internals of the Profile Manager web app (it's built with Rails) and delete the cached profiles directly. I'm really uncomfortable going into production with that as a workaround.
Note that I'm just beginning to test this, and I rather strongly recommend providing feedback to Apple rather than hacking the guts of this moderately complex system yourself.