Yup, just switch the bulbs out and change to 12/12. It may take up to 2 weeks or so to fully switch to flower but no other steps need to be taken. You can clone at any time, but taking a clone from a flowering plant will itself be in flower and take a few weeks to revert back to a veg state. It may also take longer to root if it was taken while in flower. Cloning right before putting it into flower will be fine.
As far as yield goes, there are a LOT of factors that effect the final harvest weight. However, you can pull a few oz off a 24in plant if it all goes right. Good luck man.