Here are the answers to your questions, from easiest to hardest. :)
If I leave a HSA plan to join a PPO in the future does my saved money vanish?
No, your money does not vanish. Your HSA is yours to keep. Even if you become ineligible to contribute to an HSA in the future, you keep your account, and you can withdraw on it for eligible medical expenses.
Do I run the risk of my employer changing the HSA terms in the future to make it less favorable once we don’t have the choice of a PPO anymore?
This could happen at anytime with any plan; it is not unique to this HSA plan. Every year the insurance company changes the rates and plans that it offers to your employer, and your employer has to decide which plans to offer. The rates from the insurance company are out of your employers hands, but the employer will then decide how much they will pay and how much will be taken out of your paycheck.
Is this HSA plan better for me?
For this, you will have to take a close look at the plans and look at your family's medical situation. I recently wrote another answer that outlines how to do this.
It looks like your gut is telling you that on the surface, the HSA plan is better. You might be right about this, but run some numbers a few different ways to be sure. However, if you choose to keep the PPO plan, you are just putting off the inevitable, as your employer has told you that the PPO plan will only be around for one more year.
I see that there is a $24 per year fee for the HSA account that they want to sign up for you. It's unfortunate, but it's not really that bad. The company is contributing $500 a year to your HSA, so just think of it as a $476 a year contribution. If you leave the company, or switch to a non-HSA plan in the future, you can roll your HSA over into a new HSA with a different provider that has lower fees, if you want.
Why are the HSA terms so good they seem to be really pushing it; how does it benefit my company?
HSA/HDHP plans have a number of advantages over PPO plans. The premiums are less, because the deductible is higher. You are paying more out of pocket, but this is offset by the lower premiums and the tax advantages of the HSA.
By paying for things out of pocket, you are taking some responsibility for your own health care usage. People tend not to go to the doctor quite as often when it costs them money out of pocket. With the PPO plan, you've already paid for the doctor out of your higher premiums, so you have an incentive to use it. Since, with the HSA, you have an incentive not to use it, the medical costs are kept down.
The prices you see are only the premiums that will be taken out of your check. However, the company also pays some of the premium, and I'm sure that they are saving some money with the HSA plan as well.