Generally speaking, is it possible to change the tint in a lens without needing to do new whole new Rx sunglasses lenses? And is it easier with plastic lenses versus glass?
Generally, for uncoated CR-39 & uncoated hi-index (up to 1.67, but excluding polycarb) the tint can be neutralized in the tint unit, sort of bleached, then re-tinted a different color without a lens change.
It depends on the age of the Rx. If it's older than 2 years, they may not want to dispense an expired Rx to you, even if you're bringing the original lens to them.