The company will probably say that this is "company offered" which can be changed and modified at the discretion of the company. They will "ask" for your input, (probably a survey or two) but the airline has complete control of this benefit.
I don't think that the Unions can do a whole lot about this one. The company may let the unions hash it out in some form, but in the end it the company will have the final say. This is company offered, and they can control it any way that they want it. I remember in my case, the company gave us "surveys", and in the end, the retirees lost their first boarding privilege before actives. The sUA people blamed sCO for this, but you have to remember, there are more sUA than sCO, and obviously active people on both sides of the house voted in their survey that actives take priority than retirees. (Some people think that they won't be around to retire, so get their flying in while they can now - and with what is going on now, they are probably right!)
Either way this turns out, someone will be unhappy........