Is energy required to form bonds [phase change]
Bond formation always lowers the energy of the system (or bond formation is a consequence of lower energy, take your pick.)

Indeed, you may have to add energy, because presumably certain bonds must break in order to rearrange the atoms.

Adding energy won't guarantee formation of much higher energy isomers, because the atoms have a large amount of kinetic energy, and so they can just as easily turn around and go back whence they came.

Sometimes the kinetic energy can be dissipated by solvent, collisions with inert gasses, etc. and then you may end up with a measurable quantity of the higher energy species.