With each milestone there will be new perks that have not been on the old ones. But expect some perks to carry over each time for basic rewards as new funding crowds show interest. Even if the tiers they show up on may change, I'd feel confident to assume a copy of the finished game (and amount of copies), emote packs, beta exe access, things of that nature will show up again and again. The changes you'll see will be in titles, skins and such.
Of course Grummz and Ronyn can step in to correct me.