Upgrade from WEP to WPA/TKIP...???

How difficult is it to migrate a cisco wireless network from WEP to WPA?

What kind of issues could I expect...the clients connecting are constantly changing, so any and all Operating Systems, hardware platforms, wireless cards, wireless hubs, etc. will be connecting.

How difficult can a migration like this get? My biggest concern is if we have several customers unable to connect we cannot just tell them oh well they don't have access, or go buy a new cpu. Are there any easy fixes/workarounds for this? If not, any ideas of where I can get an idea of what legacy OS, hardware platforms, cpu models, etc. do not support WPA?

Thanks!

Anybody...??? Somebody...???