Quick question about light times.

Right now I'm 12/12, noon to midnight. I'd like to change it to 9 am to 9 pm instead and I was wondering if I should try to change it an hour at a time or if it would be ok to do it all in one go?

just reset your timer to 9am after the end of the light cycle @12am.... a few extra hours of light shouldnt hurt anything in a single 24 hour period. its only when you have light cycle/ light leak issues for a few days in a row that you need to start worrying about hermie issues.....