Can I turn auto-shrink off while on production server with people connected?
I have to admit I expected this to require a lock on the database which would mean you couldn't do it while someone else was in the database.  But just to be certain I did some testing and was able to update the setting while I had an open transaction on a table in the database.

So technically yes you could make the change during the day.  However, just because you can doesn't mean you should.  In general any changes like that should wait until after hours (or a maintenance window).  Unless of course auto_shrink is causing you production issues that you don't feel like you can wait on.  

It's definitely a good idea to turn it off as soon as you can.  There are a few edge cases I've heard of where it's helpful but not very many.