My move is also to an Openserve section - moving from Frogfoot. There is already an ONT but not sure of the state of the connection... at least I have Rain 5G as a backup which SHOULD work
If the existing B number is ceased Openserve will likely swap the ONT. If the B number is not ceased and the B number is in the holding pool even if you don't know the B number when the ISP places a provide order the B number will be migrated. If the B number is still active and you place a new order it will trigger installation again.