Your dealer should've informed you about this. Anyway, I don't know what happens if you downgrade - I mean you got a refund (because 8 is more expensive than 7, almost double price because of Application Builder [AAB]) or what. But i'm not 100% sure, that a "downgrade" is officially possible for ArcPad. I don't know about ESRI's policy according to this, but you can ask your dealer or ESRI about it.
Pre 8 ArcPad versions have a separate Application Builder package that you can purchase. This gives you 1 license for both AAB and ArcPad and 1 year maintenance for AAB.
Hope this helps
Zoltan