1) It depends very much on the system of high magic you have in mind. Golden Dawn magic, for example, is primarily astral, but the high magic of Franz Bardon is at least as much focused on the etheric plane as the astral. What makes something high magic isn't the plane it works on, but...

2)...the goal, which is indeed the transformation of oneself. The opposite of high magic isn't "low magic," by the way, any more than the opposite of fine arts is "coarse arts" -- it's fine arts and performing arts, and its high magic and practical magic.