Changing from 18 to 24 hour cycle during vegetative growth.

I think the title says it all. Does it hurt for a plant in vegatative mode to be taken from an 18 hour room to a 24 hour room? This has already been done to a plant that will be used as a mother. It was an immediate transition. The plant has obviously shown preflowers. It was taken from a 250 watt HPS to 3 40W CFLs.

I searched and could not find any info about this, you might imagine how such a threat might be hard to search for because all I get is 18 vs 24, not 18 TO 24.

Thanks for any advice!