Changing the water before the safe start has come out of its dormant state and established in your filter, will cause it to fail.
You should not start testing during the 2 weeks so you don't panic. I would wait out the whole 2 weeks before deciding whether or not to add another bottle...
My son starts school next Thursday. When I was in school, the first day was always the day after labor day. He's not complaining though, for three years I sent him to a year round school this was his first full summer break.