When will these wealthy Trump counties go Dem?

[Skill and Chance]

Kendall, Frederick and Morris should flip in 2020, even if Trump improves nationally to win the PV.

Most of the non-bold ones, along with Collin, TX and Carver, MN should flip in 2024, which should either be an 8 year itch election or an easy race for a 1st term Dem incumbent.  The big exceptions here are all in the NYC area. Suffolk and Putnam, NY and Monmouth, NJ, which seem to actually like Trumpism more than generic Republican.

As for the bold ones, other than the aforementioned Collin and Carver, which swung like heck away from Trump last year, they should be voting R for many decades.  In the long run, Calvert, MD and York, VA could eventually be overrun with government employees (in the latter case from Hampton Roads, not DC), but the margin is quite a lot in both and Calvert swung hard to Trump.

[Skill and Chance]

Something to keep in mind here is that we are looking at countywide data, which only goes from a bit under 1/2 the national median income in the poorest counties to a bit over 2X the national median income in the richest counties.  A lot of the errors in contemporary political analysis come from treating 1/2 median income households as "the poor" and 2X median income households as "the rich" when the federal poverty line runs from about 1/5th to 1/3rd of the median income (approx. $12-20K) for most households and what most people think of as the cut off for "the rich" hovers around 5X median income (approx. $250K).  So when we look at the county level, we aren't really getting a good sense of "the rich" or "the poor."

My sense is that "the poor" in the sense of the federal definition of poverty still vote 2:1 or 3:1 Dem consistently, while "the rich" in the sense of $250K+ never voted Dem since LBJ and then swung to a a near tie for Obama 2008, swung back to Romney 2012 and then swung really hard to Clinton in 2016.  You have to look at the town or zip code level to see this, though.  Think Atherton and Palos Verdes, CA.  When you look at aggregate data, all you can see for "the rich" is the tail end of the $50-$150K GOP base, which greatly obscures the shift toward a $40-$100K GOP base under Trump.