I highly doubt that. This is because the % of students who get 8's and 9's is similar/the same as those who got A*s. It's all about the percentages. For an 8 to drop down to an A, more people would, therefore, be awarded 9's to even it out if that makes sense. Also, I doubt most med schools are that picky. However, it varies from Uni to uni so it's worth looking at the ones you're interested in. If anything like that were to happen, it would more likely be the University changing its individual requirements as opposed to exam boards lowering the quality of a grade 8