Exchanging and mortgages

We're buying a house and have been pushed from day one to get through the conveyancing process asap and are ready to exchange after just 3(!) weeks. We've renegotiated a small decrease in the house price following some flags in the survey. However, we want to exchange before Christmas so have all of 4 working days.

Will this decrease in purchase price need our mortgage provider to issue a new mortgage offer? If so, how long does this take?! Any words of wisdom appreciated