Dave (and others) Endicott Arm (and Dawes Glacier) is the other half of Holkam Bay, and in very close proximity to Tracy Arm. That's an easy substitution.
Hubbard Glacier is a bit of a different story. For one, there's not much else nearby, so a quick substitution if it's blocked is unlikely without several days of re-planning, port re-scheduling, etc.
Also, someone asked why it was blocked. Hubbard is advancing so rapidly (again) that one side of the glacier has almost completely blocked access to the bay. This isn't just heavy calving activity where the icebergs will clear with the next tide, it's an actual solid wall of ice and dirt pushing its way across the entrance to the bay. If the entrance is mostly blocked, the large cruise ships can't enter, and the most you'll get to see is a small corner of the glacier from several miles off. The real fear, however, is that if the bay is completely blocked, it will cause the sea water (salt) to spill over into the nearby Situk River (fresh) which would be catastrophic to the river, salmon, and life that depends on a clean river.
Better not for now... signatures get me banned!