There is an upgrade path in Australia for the GNS430 to GNS430W. I think it was discontinued in the US, but if you look at Aviation Trader at least Complete Avionics offer an upgrade for the 430. I'm not sure if this is available for the 530. I believe the upgrade involves new circuit boards.
As previously noted, WAAS requires a different antennae & cabling.
The new Avidyne GPS units are also slide out / slide in replacements for the Garmin 430 & 530