Afaik
Rahxephon Tagen Hensoukyoku is the RahXephon
movie which is already out in Japan AND licensed as well.
Now as to your original question... let me see if I can find some info...
Update 1
Ok... now like I mentioned, here is some info on the movie:
http://www.cdjapan.co.jp/detailview.html?KEY=ZMBZ-1594
(note that it mentions "theatrical feature")
The only info I can find about this OVA is this though:
http://www.cdjapan.co.jp/detailview.html?KEY=ZMBZ-1400
However, this OVA is named "A Prelude" and not "Interlude"
and was released about 2 months after the first episode aired
in Japan -- in March of 2002.
Update 2
(removed)
Update 3
First: Ignore previous comment above. Second: merged threads.