How can you change traditional plans & orientations in marketing & sales after sudden development in market trends for a specific product & service ?

Actually the marketing and sales managements set up their plans and orientations to get some progress in the marketing and sales activities for their products and services. Sometimes, occur unexpected changes in the market trends which oblige both managements to adopt new alternatives and solutions to encounter this situation and reducing negative impacts on the their marketing and sales. Kindly could you illustrate how they can change their set plans and orientations by taking tangible practical steps to face a such critical case immediately.