They use different frequencies, so there is no direct upgrade path.
Even if you open the AP and replaced the PCMCIA card you will not get 802.11a (.11a cards are CardBus and all of the parameters are different, plus the only shipping products use the Atheros chipset, which the Aironet APs were never designed to talk to.)
The only currently shipping APs that promise an upgrade path are those that use an external slot (you can see the PCMCIA card sticking out) and I don't think that their upgrade is shipping - just the press release.
