Did you talk to a Garmin representative on the issue? When I was looking into WAAS upgrades they offered trade-ins for 430 <-> 650, which in the end were less expensive than the 430 -> 430W+STC due to the AML mechanism for the 650.
In my opinion the 650 is a poor choice as an upgrade. The screen is too small especially if you want to use the touchscreen function. For my money I'd go for the Avidyne IFD440, it's an up to date, state of the art, plug and play replacement for the 430 and 430W