AD Exchange General – Delivery options is what you would use. Set the forward to address to the new employee. You do not need to do any more, but it may take some time to replicate.
Adding the old address in the delivery options will not work because the old account is disabled and therefore cannot be forwarded from. Yes you will need to reconnect the account and grant the new user access. That is what we do in these cases until we are sure the old account is no longer needed. Then we disable it again. You can also turn on out of office on the old email account and include in the out of office message to contact the new employee for future correspondence. You could also do the forwarding in the delivery options once the account is reconnected. It will work then.