> Is this a case of people over-hyping the problem...

As best I can tell, this is the case. The price of helium has gone up quite a bit, but non of the commentary links that to the actual production volumes of helium or size of remaining reserves.

> Can you just put hydrogen into an existing helium blimp

We'd need to modify the envelope design, but the rest of the design wouldn't need to be modified (aerodynamics, controls, propulsion, etc.)