Hi Diane,
With the existing set of features and functionalities, this is a product limitation. We are well aware of inconvenience this and inability to delete users limitations are causing to our customers, and we have enhancement requests filed for this:
CSCuq28629 New user not able to modify meetings from outlook
CSCuo41884 CWMS login using LDAP ID instead of e-mail ID
CSCub27578 Ability to delete user accounts in CWMS
Please, share these with your Cisco Account Team to push with Product Management to include in the upcoming releases.
As for the workaround, if you have the ability to re-enable the old account and make the host add the new user profile as an alternate hosts to scheduled meetings, that is the only thing other than rescheduling all the meetings I could suggest.
-Dejan