I'm kind of confused as to why you want to go back to an 18/6 light cycle.

If your plant is five weeks old and has been on 12/12 for a while, it's probably just starting to get ready to flower. Depending on the strain, you could be harvesting in 8 weeks or so. If you switch back to 18/6 your plant will stop flowering and go back into veg mode, delaying your harvest.

Additionally, for an indoor grow five weeks of veg time is usually just about the most you want to give your plant. It will double or even triple in size as it flowers and then the colas grow on top of that. If you let it veg for much longer your plant may end up too big for your growing environment.