Can't we just taper it "on" and see what the effects are? For instance, in the first year everybody gets 10% UBI, second year 20% UBI, etc. until 100% UBI.
If by year 10 the economy still functions as desired, we keep UBI. Otherwise we taper it off again.
The only way for this approach to work would be to define the metrics of "success" before starting this taper process. Also, it would need to be implemented in such a way that makes the process unstoppable.
Otherwise, it will be nearly impossible to taper "off" in the event of a failure.
It's easy to start giving people money that they never had, but once they have a taste, it's going to be hard to take it away.
I also think there's never been a pattern of having "success metrics" tied to laws. Laws are LAW. The best we could get using current legal technology would be a built-in expiration of the law. And if the people think it's good, then we'd vote for representatives who will retain it.
If by year 10 the economy still functions as desired, we keep UBI. Otherwise we taper it off again.