You can do it in 15 or 30 minute increments, depending on your timer. Extend the night length, then extend the day length:
Day 0 - On 5pm Off 5am
Day 1 - On 5:30 Off 5:30
Day 2 - On 6am Off 6pm...
Essentially you will be shifting your cycles incrementally to avoid any excess stress. If I were you I would seriously think about going 11/13 when you have completed your 7 hour shift. Each day you make the shift you extend your night cycle by 1/2 hour, which means your plants will be on a 12on/12.5off schedule. Making the final transition to 11/13 would only be a matter of extending the night cycle one last time. The entire process will take 2 weeks to complete.
M.