Within that context, negative income tax was sought to replace welfare, social security, Medicaid, etc. Is that the case with UBI?
Now, those programs are complicated, involve a lot of overhead, often underutilized, and create unintentional disincentives to work when the "marginal tax rate" (in taxes or lost benefits) sometimes goes over 100%. Offering a simple & big-enough UBI instead can fix all that.
(A UBI can also replace minimum-wage laws and most if not all unemployment insurance. And perhaps, many workplace regulations, because a citizenry where everyone can say "this job sucks, I'm outta here" can be a better check on abuses than the current system of compliance-surveillance, regulatory-fines, and court-cases.)
UBI is transparently an attempt to redistribute tax income to private companies such that no opportunity for profit stays with or is limited by government. UBI would mean no more Medicare stipulations, no more existing conditions protections. Just your ten grand a year or whatever and a bunch of private companies who have what you need, at whatever price they set.
Indexing the UBI amount to some basket-of-subsistence-consumption could likely address this concern.
(The handling of existing-conditions is a regulatory issue orthogonal from UBI. I'd prefer a UBI replace all "income-supporting" programs – including things like food stamps and housing vouchers – and also obviate many rules premised on the idea that the threat-of-privation captures people in bad jobs/relationships. But "structure-of-market" or "acceptable offerings" or "required minimum standards" regulations are a separate matter.)