Want to change my light cycle...safe?

Turn the dials on your timer to 8pm on and 8am off. Serious answer with minor sarcasm.

 

I'm going to need some backup for this because I'm not positive...

But, I think you can run them from 6am to 8am and then just keep it off for that 12 hours until 8pm... then it runs to 8am... etc.

 

As you now start 12/12 you have to remember that it is the extended dark period that triggers the plant's hormonal reaction to flower. So, once you get out of veg you don't want to extend the light period. Therefore, don't let the lights come on at 6am and then run all the way to 8am the next day, that's 26 hours straight light.

You could let the lights come on at 6am and then turn them off at 8am same day, so 2 hours light, then you are on 12/12 starting with your first dark period 8a-8p that day, or you could let the lights go off at midnight and then not turn them on again until 8pm the next day, 20 hours of dark would be fine. In fact, some growers purposely give their plants 24-72 hours of straight dark when they transition from veg to 12/12, believing that it will "kick start" the flowering reaction (this has not been proved or disproved, and if you do it or not the plant will still flower in 12/12 either way, it's just a debate about whether it gets things going a little quicker).

Just remember in flower it's OK to shorten light or extend dark, never to extend light. In veg it's just the opposite.