The Wobbe Index is an indication of interchangeability of fuels.
 
To change from a specified fuel of 42 to something less than 30 is not recommended without an engineering evaluation of burner and the associated fuel and combustion air train.

Changing the fuel to one with a lower Wobbe index will mean that the energy released per unit volume of fuel will be less than before.

This will result in the burner operating at a higher excess air rate with the same volume of fuel gas being burned.  Also, it would limit the maximum firing rate of the burner. This could result in extinguishing of the flame.
 
It is possible to change to a fuel with a lower Wobbe index.   However, this would require an evaluation of the operating parameters and then possibly changing out some of the equipment.
A final determination can be made only after a review of the equipment, operations requirements and proposed fuels.