do i need to replenish unity pro subscription after a game release?

I have a game that I want to publish and I want to use unity pro for it, because i read somewhere that if i use unity free, I can only use it if i have under a certain amount of money from sales. Is this true? And if it is, can I purchase it once (when i release it) and not have to worry about renewing it? And will I have to renew the subscription every time I want to update it? If it helps you answer the question, I plan to release on Steam Greenlight