GNSXLs
I am not current on the GNS XLs but I seem to remember the PRNAV approval is both a hardware and software upgrade.
I believe, from memory, this was required because the way the '0's and '1's bump into each other in the hardware could not support the different way point requirements. Something about flyby waypoints not being true flyby but being get close to and then turn to intercept the next track waypoints.
Try calling the honeywell UK rep directly.
You may wish to consider going straight for a GNSS upgrade as there is a EuroControl NPA out suggesting that all airports should have performance based approaches very soon. Many airports seem to be very keen to comply as it will remove their need to maintain the very expensive ground based approach aids.
Regards
MM