Can I switch my Wireless Cards after the new update at WWDC?

No, all the components are soldered onto the logic board.

Edit: Given the released photos, looks like I'm wrong, though apple will not be selling these to existing owners, so my answer is still no

I don't think the wifi card is a user replaceable part.

Is the WiFi card replaceable? Sure

Is it user serviceable? No

Also to note, the current WiFi card in rMBP has 3 antennas, so if the AC one has 4.... you will be missing 1 antenna.

(One screw holds the card in place)

not to mention that even if you could get your hands on the card there's no guarantee that the chipset would support or that there would be the proper driver.

I don't see a need for 802.11ac while I'm out and about. At home I'll use it for fast transfer of big video files between my computers. I plan to add it to my rMBP with an adapter attached to my USB3.0 hub. That will allow me to ditch the wired Ethernet connection and the nasty Thunderbolt-1GbitEthernet adapter. So, just one cable from my rMBP to the hub and I'm done.

Yeah, it would be cool to swap the internal WiFi card, but that will probably be more expensive, and it may not even be possible. We'll have to wait and see.