Donating to a game will make you "own" it, regardless of what the price is set to in the future. So if you see a free game you don't want to lose you could technically donate the minimum (50 cents). We might change this in the future, but we'll never take away any games you may have claimed.
(The only exception to this is individually priced files, those will require a donation amount above the minimum)
More information here: What happens when you convert a free game to paid