As title, I will be using my old led lights at the start if the grow, but want to switch to hps when I have established plants. Will I need ballast or not?
Yes the hps needs a ballast.
Definitely need a ballast and unless things have changed drastically since my earlier days, you’ll need the appropriate ballast for the load and type of the lamp. A MH used to require a different ballast than a HPS.
For instance, if you run across a ballast that happens to be compatible with the wattage of your lamp, make sure the ballast is a multi-tap ballast that you can wire for your voltage ( generally 120v…some guys used to run dedicated 240 drops/outlets).
Many industrial lamps people grab are setup for 3 phase power and require 277vac or 480vac. I’ve known several people that bought these lamps and then had to change out the transformer and the starter ( capacitor).