By "unassigned seat," are you referring to your original coach seat after the upgrade to Comfort+ ? Once this upgrade to Comfort+ occurs, this becomes your new assigned seat and your original coach seat is opened up. You cannot transfer between classes unless you call Delta and ask them for a better seat in coach (an aisle or emergency exit for instance).
Or are you suggesting that once you are upgraded to Comfort+, you lose that seat assignment and thus you are left with no seat at all? If this is the case, it's not normal and you should definitely call Delta for an explanation.