I am having the package unit at my townhome replaced. The HVAC contractor wants to charge me to flush lines as the old unit is R-22 and the new unit is Puron (R-410a). I asked why this was necessary and the manager explained that the refrigerants are incompatible and that the old lines have to be purged before they can be reused. This makes perfect sense to me except that I have a package unit. By its very definition it is a self contained system. I have no external components. I know just enough about all this to be dangerous so it is entirely possible that I am missing a vital piece of info. However, I think it is also possible that the technician who noted this on my work order after the fact may have confused my job for another with a multi-component system. I think the only connections I have are power, thermostat, and ducting to the intake and plenum. Am I correct in assuming that since the package unit is self contained that there should be no external refrigerant lines that would remain and require flushing?