If you do the operation yourself, you are selling your HIVE to buy HBD.

You use the collateralized_convert contract to convert, not sell, Hive into HBD.

You make a profit as long as the price of HBD does not rise, and its value remains high, while you convert.

What happens to the price of HBD after you first sell it does not affect your profit - only the price of HBD at the start, the Hive price at the end after you receive collateral and the feed price (ie. the median price reported by witnesses over 3.5 days) which determines how much collateral you actually get back affect how much profit you actually make.