Change encoding mac airport from WEP to WPA?

I live in Germany and am using a new German mac powerbook with an older American airport device bought about 2+ years ago.
All worked perfectly for years, via the phone box provided with my server here. Even when I moved to another city.
The WEP password which I first designated was immediately saved into the memory of my old mac and it even prevailed after I had to recover my whole system into my current new laptop (via Time Machine), after the theft of my old one.

Suddenly today, for the first time in over 2 years, when trying to log in to my own dsl internet network, I am told that none of my preferred networks are available. Clicking the airport icon, my network clearly appears among the list of other networks, indicating that it is locked.
Upon clicking on it, I get a window telling me that a WEP password is required.
Typing in the password which I myself designated on day 1, I am told it is incorrect.

I called technical support from the server who provides me with internet and they were very helpful, but in inspecting the bowels of my internet access setup they only found a WPA code listed. This code has nothing to do with any code I have previously used, or invented.
It seems (or so I was told) that my present telephone box is theoretically only compatible with WPA, although the airport device I have is clearly WEP oriented.
What that theory fails to explain is how everything has worked grandly for months until today. Moreover, how could my old WEP password have reinstated itself automatically when I moved and got a new mac, despite a new telephone box which would otherwise have demanded WPA?

Can anyone explain why this would suddenly happen?

...Is it possible to change from WEP to WPA, and to override a WEP password which had previously locked my network?

Thanks for your help!

NDG