What would constitute the substantial reason to change the terms? That would still be explored, but as you say, would not need to be proved.
I'm not convinced that it is in the purview of an Employment Tribunal to "explore" such a reason, it is certainly not for the ET to decide whether such a reason is substantial, the test is not about the reason itself but rather whether the company holds a genuine belief in such a reason. Even if an ET were to take on such powers to itself, it would be easily overturned at appeal - there is case law to back this up.
Importantly the ET is highly unlikely to try to rule that a person dismissed in such a fashion must be re-employed, I'm not even sure they have that power, I think the best they can do is award limited compensation.
It seems that SOSR dismissal while remaining an option is not one that BA seem keen to take. I do note that they have done enough to establish that they do have a genuinely held belief that the imposition was required.
I suspect that SOSR would only be used should someone fail to sign a new contract, offered as a take it or leave it deal.