In nature the light cycle changes over the entire course of the season. Everyday it is a little different.
I don't think switching immediately is going to stress your plants very much. I also think that slowly switching will in fact cause more stretch as the light deminishes and the plant is still not in flowering and it will stretch more and more the less light it recieves.
I am however interested in the idea of slowly changing the photoperiod over a period of a few weeks. Marijuana does not need exactly 12 hours of darkness to flower, depending on the variety some will flower under only 11 hours of dark or even 10 hours with indica dominant strains. It would be interesting to take a given mother and slowly change the photoperiod to determine exactly how many hours of darkness are needed to initiate flowering. With that knowledge you can keep your plants under more light and have a larger harvest. You would however need to do this test with each plant/mother you grow as it would be different with varying genetics.
12/12 works for almost every plant but is not the only photoperiod
It would be cool to have a cutting of a popular clone only strain and have determined that it can flower under x amount of darkness... then anyone else growing that cutting would benefit from that knowledge.
For anyone growing outdoors, noting the length of day at which the plants you are growing initiate flowering may be an easy way to determine the photoperiod requirements for your cuttings.
I've never heard of anyone straying from the 12/12 photoperiod however, so though it makes sense in theory, it is possible that there could be negative side-effects or even hermies as a result