Bob, I read the letter you forwarded from HR, and it says the company's contributions are returned if retiree plan is terminated.
As I'm reading the overview of the TA, there's a pretty good argument to be made that the retiree plan isn't actually being terminated.
The company is still offering retiree coverage at the lower "value plan" rate:
Since it is still all in a trust, it's pretty easy for them to argue that their contributions and interest are going to be applied towards subsidizing the "value plan" coverage costs.
Unless there's more complete language sitting somewhere else, the side letter clause you're invoking is targeted specifically at termination of retiree health care, not modification or substitution. I don't think it would wind up in your favor in an arbitration.
I know, it sounds like pro-management spin, but the devil really is in the details.
And, once again, it looks like you might have been caught again by the trap of not having adequately trained people negotiating your contract language.
There's a joke about why divorce is so expensive ("Because it's worth it"). I know I'm beating a dead horse, but having a competent lawyer makes all the difference years from now when you need to revisit something... The same is true about contract negotiations.