trading in for a VIIC is a good move as they have faster dives, and even more useful... a stronger pressure hull. the VIIC could generally go deeper than the VIIB use ful when the escorts begin to get good.
Also the VIIC has far more options for upgrade over the VIIB like better conning towers.
Also the speed difference is negligable at best a VIIC will do 8 knots submerged too.