Humans are rarely really still. Crocodiles beat us handily in that department (and in no-rules mixed martial arts too, but I digress). "Thinking hard" also causes lots of semi-voluntary movements.
When sitting, in a chair with back support - these same movements largely get restricted to neck, feet, arms from elbows till finger tips, slight weight shift along the bottom. When standing, new possibilities open up.
It is not unusual to see standing desk people moving feet around - this feet movement is 20 times more energy intensive than sitting feet movement because the feet are supporting much more weight. Same goes for small bends in knee and ankle joints. Arms are also now free to move much more, even backwards. Torso bends when standing are more energy intensive too. People are also motivated to not keep paper, pen, odds and ends very close to the place of work, but arrange it at some distance such that it is 2-3 small steps to fetch them rather than "making a long arm" that people typically do in a sitting desk.
So simply switching sitting to standing does switch sitting issues to standing issues, but while moving larger parts of the body more frequently. Circulation and calorie consumption increase.
And yes, as you too agree, mixing these up and adding some walking is even better.