If you make your game free now, then later add a price. The people who downloaded the game while it was free will not own it, so they will lose the ability to download it.
If you make the game paid now, but then create a 100% off sale, then the people who claim it will forever own it, regardless of what you set the price to in the future.
You should choose what you think makes the most sense for the kind of game you're making.