What is necessary? It'll pay off for us to get this on the table.
If we knew exactly, someone would have a nobel for it and the nonperson predicate would be a solved problem by now, along with the Hard Problem of Consciousness and a throng of other things currently puzzling scientists the world over.
However, we do have a general idea of the direction to take, with an example here of some of the things involved. There's still the whole debate and questions around the so-called "hard problem of consciousness", but overall it doesn't even seem as if the ability to communicate is required for consciousness or sentience, let alone hold the ability to parse language in a form remotely close to ours or that allows anything akin to an argument as humans are used to the word.
But past that point, the argument is no longer about UCMAs, and becomes about morality engines (and whether morality or something akin to it must exist in all minds), consciousness, what constitutes an 'argument' and 'being convinced', and other things humans yet understand so very little about.
Okay, I see the problem. Let's say this: within the whole of mind-space there is a subset of minds capable of morally-evaluable behavior. For all such minds, the UCMA is true. This may be a tiny fraction, but the UCMAist won't be disturbed by that: no UCMAist would insist that the UCMA is UC for minds incapable of anything relevant to morality. How does that sound?
If it's worth saying, but not worth its own post (even in Discussion), then it goes here.