If sales decline due to bad economical conditions, should the company adapt or create a new strategy and why?

The severity of the economic conditions dictates the decision to either adapt or create operating strategies. A temporary slump in market conditions due to seasonal fluctuations requires taking sufficient adaptive measures but a nation-wide economic depression can be survived only if the company stakeholders take reactive measures. 

Companies do not work alone, but in the political, economic, environmental and social variables, and any change in these factors will affect the company as a whole and not only on sales, it is obliged to change its strategy and decisions according to the new conditions.

Otherwise, as the ship will be sailing reverse waves

 Regards.

Creating a new sales or marketing strategy is a form of adaptation in the face of economic turbulence. I agree with Mahreen's impeccable answer that this should relative to how pressing, how long and how national or international these conditions are, in relation to your company's operations.