I think your amendment has to answer that question and ought to do so very clearly. You can write the amendment to give back years of service or only going forward. (See My 2 Cent's comments about discrimination and so forth).
But a vague amendment is only going to be trouble. You are going to have to constantly explain why it isn't retro if it is silent to people who didn't work 1,000 hours in the past but worked 500
This is a great example of a time you really want to think about how to word an amendment to make sure you don't create any unintended consequences.
For example is the 500 hours after 8/1 or for all of 2016? (Assuming a 12/31 PYE) I would make the amendment clear on that point. I would think it is all of 2016 but state that.