That's why, as I said in my post, BA would have included a clause that allowed them to vary the terms of ST.
If this was not the case those that left the airline with a severance package, would forever, have better terms and conditions than those still working for the airline or those who just retired.
For instance if BA changed ST to say that no one could travel in First under any circumstance the lucky old severees would continue to receive this privelidge when others would have lost it.