IIRC, the BV12 came with the BSys4? The Bsys4 should almost be interchangable with the Theatre except for the picture/monitor control. The problems I see is that :

1. The BV12/Bsys4 is a Beo4 controlled system
2. The BS Theatre is a BR1 Bluetooth control.
3. There is no native APP for the BeoApp in the Screen (and hence control)
4. There may be no tuning capability in the BV monitor (disabled/delegated to Bsys4)
5. Modern OLED Panel connectivity to DNLA, other equipment, CEC, will be missing

There may be a way to connect both the Bsys4 for monitor and IR control and have the BS Theatre together but I'd bet its very clunky? However, the tech between the two is over 10years apart (and probably 15 years between the panels)? Looking at the LG G2 (or new G3) 65, why would you connect a BV12 and miss on 50% of the interaction fun when a BSTheatre and LG is arguably the best featured Beovision ever made?