What was Dick York's last episode? What was Dick Sargent's first episode?
Why did Dick York leave?

Daddy Does His Thing seen on April.3,1969 was the last episode filmed by DY on about mid Dec./1968.
After refusing a magical gift from Maurice, Darrin is turned into a chess playing donkey.

Samantha and Darrin in Mexico City seen on April.24th, 1969 filmed on Sept.19,1968 was the last episode shown with DY on Bewitched.

When a nervous Darrin has to speak Spanish before a meeting in Mexico City, Endora casts a spell to make all his fears disappear. Unfortunately Darrin completely disappears because he is all fear.

Sam and the Beanstalk see on Sept.18,1969 was the first episode with DS, it was filmed on April.18,1969
Sam must retrieve Tabitha who has run away into the book 'Jack and the Beanstalk.'

The first ep. DS actually filmed was Samantha's Better halves filmed on March.31,1969 and see on Feb.1,1970.

As to why DY left:

http://www.harpiesbizarre.com/yorkarticle1980.htm