Keep in mind that if HIVE is converted to HBD, in theory the total market cap of the two (combined) should remain the same.

This would be true if HIVE had no other utility. However, we know people do want HIVE/HP for RCs and for voting and governance. Actually, usage of HBD increases demand for RCs. So it is likely that HIVE being converted to HBD will increase the price of the remaining HIVE. But time will tell.