I've been using potassium in my rain soft water softener. Can I switch to salt now, without damaging the unit, since potassium is so expensive?

Should work unless your softener was designed with a potassium specific ion exchange resin. Chemically PCL and NACL are very close and are neutral salts. You can change them but may have to tweek the amounts used. Why were you using Potassium Chloride?

The salesman told us to use it. He made it sound like the salt would be bad because of high blood pressure problems. We have a reverse osmosis drinking system. Doesn't that take the sodium out of the drinking and cooking water?

And no doubt, the salesman made more money on the KCL-I put Phosphorus(P) my bad! Yes there is some salt in the water unit but with an RO you eliminate the sodium. Use salt and save big $$