I think this is going to be a nigh insurmountable hurdle. To be upfront, I do not support UBI and I am extremely skeptical of it. But plenty of my friends on 'the left' adore the idea.
I don't believe the same people who champion a UBI would suddenly become cold and say to a disabled recipient, "No, you will not get extra money because of your special needs. You will get nothing else because we dismantled your welfare benefits in order to give everyone a UBI."
The response, then, would to be adjust the UBI payment amount for individuals with special needs to make up for the loss of those other welfare payments they needed to live. But to do that you would still need some sort of bureaucracy in order to assess claims and increase the payout. So in the end, can you even say you've replaced welfare with a UBI?