The precautionary principle is the idea that one should not undertake a new action until the ramifications of that action are well understood. for what reason would we apply the precautionary principle to switching to a hydrogen economy?

There are concerns that hydrogen gas could deplete the ozone layer. 
On the off chance that hydrogen gets on as a 'non-polluting' fuel for vitality creation, spills from its generation and transport could expand the measure of the gas in the climate. This change would exacerbate ozone depletion.