You didn't murder anything. Even the best cared for reefs have losses. Learn and move on.
Right now, I would just do the water changes and otherwise let it be for a few weeks. Slowly switch over the the MH. Then, a few weeks later, if all is still good, think about adding at that time.
And, I know you have had that tang for a while, but I would sell it. The food and the poop it makes is hard on your small tank.