It's hard to quantify since the counties with the most drastic swings to the left are those with the most drastic demographic changes. Remember that Bush's elections are 20 years in retrospect, and many of these counties are very different places from what they were in the early 2000s. The best candidates for the counties with the most Bush-Biden voters are those Democratic-trending counties which are still largely white today and aren't 50% larger population-wise than in 2000. Some are still overwhelmingly Republican, just not as much as they were back then. The WOW counties, Kent/Ottawa/Oakland MI, Chester/Centre PA, Johnson KS, Oklahoma OK, Hamilton IN, and Delaware OH are all good possibilities.
W'04 -> Biden'20 voters aren't that common, though they certainly do exist. What happens more often is that their kids vote for Biden.
