The welfare plans that might be subject to being replaced only amount to a few percent of the federal budget. The bulk of the budget is those plans above, which would result in less benefits if replaced. You're not paying for this by finding lost coins in the couch cushions.
Medicaid and Medicare being wholly replaced by a truly universal plan unrelated to UBI is probably a prerequisite for a UBI for a bunch of reasons. Social security would probably be unaffected, at least initially, and SSI/disability would likely either be reduced or gradually phased out, depending on the amounts.
I know the other various benefits aren't huge in the budget, but they add significant bureaucratic burden to both the state and the individual. Eliminating that would be a big benefit of UBI.
This has been discussed for decades. The US healthcare system is one of the biggest impediments to other forms of governmental change.
No one serious is suggesting "everyone gets $1000/month and literally nothing else changes".