I know this is an old post, but I wanted to reply with the method that works the best for us. Instead of using ConvertPStyles, we begin with a fresh drawing that is already STB based. We then insert the old drawing into the STB drawing and merge the old CTB layers into previously setup STB layers which already had the Plot Styles and lineweights associated to them. This can be easily scripted if you had good layer management when you drawings were originally created. It's not a full-on batch convert, but it works well for us.