This will make the auto attendant answer calls right away, but won't change the message itself of the auto attendant. As far as I know, that is based on the schedule of the greetings and cannot be changed with just a button push. (So I convince my customers that it's ok to do what crowtalks is saying to do)
Also, if doing this, you will be losing Caller-ID capabilities during the night service. (Atleast in this area, caller-id is sent between the first 2 rings so if voicemail answers automatically, bye bye caller-ID) Alot of people are ok with this however.
Just some extra info on Crowtalks excellent instructions...