As long as I know the plant is NOT yellowing because of a neut problem/root problem.
Any time you start training a plant "late" you need to be very careful, and go slowly with it. IF you happen to "kink" a top when trying to "bend" it to fast, you can use something like popcycle sticks & tape to mend it together until it gets strong again, and it WILL.
Just take it slow, and make sure you loosen the ties when necessary.
Good luck.
lil
