There was a licensed component in a V2 phone that is not used in V3. This means V3->V2 downgrade is blocked for a phone which was originally supplied as V3, because if it wasn't, there would be a license contravention . The phone can go V2->V3->V2 (license originally paid for) but it can't go V3->V2 (license never paid for as not required).
Edit - just noticed you are actually going HFA-SIP not HFA-HFA so this restriction likely doesn’t apply…… I’ll leave the post there anyway.