The stock expansion valve has a metal screen that would have stopped most of the debris from circulating through your system. You're replacing it but perform an inspection to see if there is any metallic debris which would warrant a really thorough flush.
Since AC flush is expensive, I use lacquer thinner as an initial flush and then use the AC flush in a final cleaning. The POA can be flushed while connected to the evaporator or stand alone. I recommend that the POA be bench tested before installation and cleaning since the refrigerant and oil may have been burnt & contaminated causing the POA to stick. You can plot the input pressure vs output pressure and note the point the POA regulates the pressure. A bad one will show not regulation and the output will follow the input pressure.
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