Changing the primary SMTP e-mail address in Exchange system is as easy as select a e-mail address & set as reply in E-Mail Addresses tab in Mailbox properties. Well, now it’s greyed out. Baffled? Don’t be.
Answer: Uncheck the option for Automatically update e-mail addresses based on e-mail address policy. That’s it. E-mail address policy is being applied to this mailbox, that’s why you can’t change the default settings.