OK, I’ll chime in here, even though I know nothing about your room, listening tastes, etc. I have auditioned the PL500 & have listened to your Proacs (and the newer big Proacs as well). As you know, these are very different speakers—physically as well as sonically.
a) The Monitors are large and need a lot more real-estate than your Proacs
b) you will probably find the Monitor tweet a bit hot on some frequencies (6-8kHz?) and less natural than the ribbon on yr Proacs
c) the Monitors have more authority and provide more volume (not amplitude) than your Proacs (no surprise) and if you listen to large orchestras, the Monitors will give a better sense of scale; bass reproduction is of course much bolder.
d) the Monitors are euphonic, pleasant, while your Proacs retain some "mastering" dynamics i.e. the Proacs will probably sound slightly more dynamic to you.
Overall, the PL500 are much more imposing than your Proacs, physically & sonically, but their sonic signature is different—you’d probably call it "more polite" by comparison.