Going to 18/6 at the End of Flowering?

Does anyone know what would happen if someone switched the light schedule from 12/12 to 18/6 at around day 60 for the last stretch of flower? Will this cause deadly shock, or anything else that you'd be concerned about? I'm hoping someone who has done this is lurking about...thanks in advance!

No harm. the plant will go back to its vegging state.
It will take a month or so to change, probably start putting out single blade leaf, then 3 blade then back to full size.

Some people harvest the top buds and leave the lower stuff and re grow there plant.

unless you are looking to reveg I would lower the amount of light say go from 12/12 to 10/14off or even 8/16off

at the end of flowering you can switch to up to 14/10 to get fatter buds with the extra sunlight but flowering time is prolonged a bit............by switching to 10/14 or 8/16 you can speed up ripening time of the bud at the end of flowering
this can be done within the last two to three weeks of flowering