I called Sprint up and they said if already are eligible for an upgrade you can keep it even past the changes, but if the changes happen before your eligibility, then you are screwed. I myself will be demoted to Silver, but I and all other launch day Evo buyers will be eligible April 1st to upgrade (go check)!
I asked if my upgrade was April 1st, and the changes were happening April 1st, and I was told the changes won't be finished until April 2nd, and I will be able to keep my upgrade. I asked again just to check that I would not have to use my upgrade April 1st, but instead could keep it and use it whenever. She confirmed that yes, I would be able to use the upgrade anytime I wanted even past April.