If it fails you can always revert to the current failed system.
In my country, UBI is supposed to go hand in hand with dismantling all social welfare, subsidies, whathaveyou. Once all the laws have been changed, and the civil service has been completely reorganized, it's a titanic fight to get them reverted, and after that, all the institutions still have to be put back in place. It would take 20 years.
There are no hard rules on structure of other programs in relation to ubi. It would be foolish to remove support for special needs etc. Unemployment benefits, public pensions etc can go away but the rest must be looked at on a case by case basis.