Originally Posted by
Ninthace
Gas balloons can gain height by dropping ballast and lose height by venting gas but they still go downwind and there is a limit to how many times it can be done. Moreover, ballast is carried at the expense of payload.
That would have been true a few decades back, but if you have onboard power (e.g. solar) , then you can use pumps and valves to inflate or deflate a separate bladder in order to adjust the buoyancy (and therefore control altitude) indefinitely. These are known as "air ballast" systems and have been used in research balloons since at least the turn of the century.