Anyone logged in as a domain administrator should be able to change the password of this account.
Would not recommend installation during business hours unless you KNOW that installation will not require either a reboot or the stopping of Exchange services or both. Also, there is never any guarentee that a software installation will work the first time. If it doesn't, how long will your users tolerate any down time (a failed install could keep Exchange from working properly).
Your best bet:
Make sure you have a GOOD backup of your Exchange installation before attempting to install the new software.
Conduct the installation after business hours (or a weekend). This will give you some time to sort out any problems that may arise. You may not have any problems at all. On the other hand, it could keep you from having to face your boss with the message "Sorry, we ran into problems. It will take us 4 hours to reinstall and restore Exchange." Messages like this don't tend tobode well for job security.