Don't a lot of welfare payments have strings attached, with regard to what you can spend it on? UBI explicitly does not, so you can use it for anything you want. If UBI did completely replace the social safety net, and someone gambles/fritters away their basic income and can't get a job to make more, are they allowed to starve (or depend completely on private charity)?
As everyone gets UBI, at first glance it might seem that the poor will get a smaller piece of the cake, but in reality a person with a good income will just give it back through taxes.
Of course, some specific people might end up being worse off (like a person who gets $30k of welfare), but on average poor people should be better off under UBI. Mainly because they can work and not lose anything.