Welcome to ATP!
First question: I don't think your Tabebuia trees are tall enough to branch. If it gets to 10 feet without branching, then you may need to "encourage" it.
Second question: plants grow from the tips: root tips and branch tips. If your trees have better growing conditions now (better light, bigger pot, new potting soil), the growth will be healthier. The entire trunk will slowly thicken in response to more growth on top and more root growth.
Daisy
