If an effect grants a creature an ability after Dress Down has entered the battlefield, it won't lose that ability. For example, if a land becomes a creature while Dress Down is on the battlefield, it will still gain any abilities given to it by the effect that animated it. It will, however, lose any abilities it already had.
If an effect causes Dress Down to become a creature, it will lose its own abilities along with all other creatures. This includes the triggered ability that causes it to be sacrificed at the beginning of the end step.