I understand it as it's an opt-in. "If you like your welfare, you can keep your welfare." So yeah, someone getting $1300 in benefits could keep getting the benefits instead of opting to take UBI. Someone getting $500 in benefits, could take UBI and just get $1000/month and no benefits. I think overtime you would see less reliance on the traditional safety net benefits as people going through hard times would just stick with UBI rather than deal with stigma of added benefits.
Anyway, the details would obviously be sorted out. I think the biggest contribution Yang is making is "look, we've got a huge fucking problem coming and we need seemingly radical solutions to deal with it." While I think there's a chance over a generation or two the economy adapts back to full employment, in the short term you're going to have a ton of people without a way to earn an income who formerly worked for an income. It's going to be a huge mess at a time when GDP continues to soar. You can't have millions of people suddenly impoverished. I think his prediction of riots is dead on, though the riots might take different form than outward violence such as skyrocketing suicide, drug addiction, domestic violence etc. This isn't going to go well.