Dude- if you don't have 2 chambers... Skip the photos from now on... Anyways, flip the switch to 12/12 and don't sweat it- you'll yield a hell of a lot more by flowering those photo than you will if you don't because of the single auto...
Yes your auto yield will be reduced- but it's no deal breaker. Other option is keep vegging til the auto is done and then flip em to 12/12.
Photo growing is very cool, to be sure-but what is this hobby for you? Is it a comercial venture , or just a way to get plenty o cheap smoke that you like? If so- 2 chambers- and do not screw with photos from seed other than to find a mom- clones finish way quicker and you can grow autos alongside your mom and rooting/vegging clones. If you are into the breeding/ gardening aspect, then growing photos from seed makes more sense- and you can go with one chamber and flip the switch ever two months- do your autos while you veg, then harvest those and flower the photos- and start over after that.
You need to get a handle on your own needs and style- then figure how the plants best fit your lifestyle.