So the choice to leave the Vaikuntha planets or Goloka Vrindavana (the Kingdoms of God) is always possible because Krsna allows the individual jiva-souls to express their free-will the way they choose.

This is only possible in an open "two-way" exchange of voluntary participation, where the individual jiva-soul can choose to remain in the spiritual world serving Krsna, or enter the material world and live there without being Kṛṣṇa conscious. 

This means the individual jiva-souls always have the choice to accept, or reject Krsna/Visnu.

Srila Prabhupada - "Free will means that you can act wrongly. That is free will, unless there is chance of doing wrong or right, there is no question of free will. Where is free will then? If I act only one sided, that means I have no free will. Because we act sometimes wrongly, that means free will. Just like a thief, he knows that his stealing is bad but still he does it. That is free will. He cannot check his greediness, so in spite of his knowing that he is doing wrong thing—he will be punished, he knows; he has seen another thief, he was punished, he was put into prison— everything he knows, but still he steals. Why? Misuse of free will." (Talk on Rene Descartes philosophy with Hayagriva dasa 1973)

Unless there is the misuse of free will allowed, even on the Vaikuntha planets of Visnu, and Krsna’s Abode of Goloka-Vrindavana, there is no question of free will.

For loving exchanges, reciprocation and cooperation to truly exist between Krsna and the individual jiva-souls, the individual jiva-souls must also have the right to voluntarily express themselves one way or the other, which means having the choice (free will) to leave the Vaikuntha planets and Goloka Vrindavana at anytime they choose, and enter the temporary material world to live their separate existence without Kṛṣṇa or Visnu.**