The original rigid airships had a series of bladders containing the lifting gas. These bladders were effectively open bottomed. As the air pressure dropped they simply let the lifting gas vent out of the bottom of the bladder. I thought this was pretty crazy when I read about it, and it did lead to a nasty loss of an early airship. The windscreen of the open top cockpit/gondola created a vortex that trapped the venting hydrogen. This eventually led to a fire/explosion and loss of the airship. Although not as crazy as the really early dirigibles before they mastered mass production of hydrogen. They used coal seam gas as the lifting gas, and even ran the engines from a feed from the balloon.
I would imagine to preserve the helium the Airlander 10 could use a compressor to store the helium in tanks and reduce the internal bladder pressure.
"Don't think; let the machine do it for you!" -- E. C. Berkeley