A minimum wage raises the floor for workers making below the minimum. A basic income raises the floor for all. The former results is fewer steps to climb for someone at the bottom to reach your floor, the latter results in an unchanged amount of steps but the stairs just happen to be located further from sea level.
If workers leave walmart to live off of basic income, all walmart has to do is increase prices and workers would again be forced to work for basic sustenance, as they would no longer afford walmart's monopolized goods. If they live in a left-leaning or urban area, they may be lucky to have a food co-op, but those are already unlikely to be paying bare minimum wage, and are generally located in more expensive areas.
At best, UBI will totally ineffective. At worst, few will capitalize on it early on to reach some success before the market adjusts, and the Koch/Cato ilk will use that success to further push an anti-labor agenda until the system implodes again.
I was all for UBI when I first heard of it, but more and more I realize it's just another neoliberal economic policy that fails to address the root causes of class division. Perhaps it will be our decade's "microloan" scam.