I rarely sweat copper pipe but did so recently. I bought lead free solder because the solder I had on hand (from long ago) had lead. The flux I found on the web said "lead free". That leads me to ask, Is it ok to use my old flux from lead solder days or do I need to buy the "lead free flux"?
Silver solder requires / benefits from a more aggressive flux. But since this is plumbing, not electronics, your old flux is probably already an aggressive flux, that works well with exposed metalwork or with the slightly higher temperatures of silver solder.
Your old flux and your lead-free flux are probably the same thing. Read the label.
I would not trust an old container of flux from the lead solder days. Some flux did have enough solder mixed in to "tin" the joint. So it would be possible that the old flux would have lead solder in it.
Get a new container of flux which is formulated to work with the lead free solder that you buy. The last solder I got five or more years ago was 97 % tin - 3 % copper. The link below references a flux which is presumably formulated to work with tin-copper solder.