Those who served our country and are facing a disability because of it will continue to receive their benefits on top of receiving the $1,000/month.
Social Security retirement benefits stack with the $1,000/month UBI. Since it is a benefit that people pay into throughout their lives, that money is properly viewed as belonging to them, and they shouldn’t need to choose.
SSDI is based on earned work credits. SSI is a means-tested program. You can collect both SSDI and the $1,000/month UBI. Most people who are legally disabled receive both SSDI and SSI. Under the $1,000/month UBI, those who are legally disabled would have a choice between collecting SSDI and the $1,000/month UBI, or collecting SSDI and SSI, whichever is more generous.
Even some people who receive more than $1,000 a month in SSI would choose to take the $1,000/month UBI because it has no preconditions. This $1,000/month UBI removes these requirements and guarantees an income, regardless of other factors.