I envision a system like this:

Person A has just graduated from high school and has never worked right when the UBI program goes into effect. He never pays into SS and receives only UBI for life and will never be eligible to collect SS.

Person B has worked under the existing system for n years and has paid into SS a significant amount. He stops paying into SS now and begins receiving UBI as well. When he reaches retirement age, he collects either SS or UBI, whichever is greater, based on the points he earned from paying payroll taxes.

Person C is a current typical retiree. He continues to collect SS, as it pays better than UBI since he paid so much into it from his payroll taxes.

This UBI system would pay out less benefits than someone with maxed out SS benefits gets today, but we wouldn't have the payroll tax anymore. Instead, the UBI program would be funded through income tax.

This would cost significantly more than today's wealth redistribution programs even added together (setting aside Medicare/Medicaid which are kind of a separate issue), so it would certainly require higher taxes to support it.