"I am pondering open figures in waltz and looking for an explanation of why the third step of, for example, an outside change, is "up" and not "continue to rise"."
Two reasons:
1) The overall rise in a foot closure waltz figure is higher, because the energy that was in horizontal movement will be almost completely converted into body altitude, letting your travel pause at the top. In something like foxtrot you'd get only partial conversion, so the body would slow but not stop, and the altitude would be less. A passing figure in waltz is somewhere in between a closing figure in waltz and a passing figure in foxtrot in this respect.
2) A major reason why a continuation of rise on step 3 is listed for the foot closure waltz figures is that the formal boundary line between step 2 and step 3 falls earlier in the sequence list of actions in the closing case than it does in the passing case. The end of step 2 for a passing figure is defined to occur when the feet pass, so any rise occurring before this is part of step 2. But the end of step two for a closing figure is defined to occur when the foot is halfway closed, so any rise occurring between the time when the feet are halfway closed and the time when they pass is formally considered to occur during step three. Thus rise occurring during the later half of the closing action is formally listed as part of step 3 if the feet are going to stop closed, but would have to be considered part of step 2 if the foot is going to pass through closure to a new position.