2 Week Old Plants Help

ive grown some plants from seed for 2 weeks on a 20 on 4 off light cycle
under 400 watt hps they have been growing well i am just wondering if
it will stress them if i switch to 24hr on??

there are differing opinions on which is best (24/0, 20/4, 18/6) but i think we agree that switching between them isn't harmful.

once you switch to 12/12 you want to make sure no light gets to your plants during the 12 hour dark period.