Plumbers have been using "lead free" solder since the mid 70's.
I switched sooner than most because of the lead issue and the fact that working around framers, you wanted the joint to cool as fast as possible.
We were getter fewer leaks with it.
If you are retiling the tub surrond, I would consider replacement of the tub.
Otherwise, perhaps reglazing would seal in the lead. Does anyone else know if it would?