There are a few 'blatant' differences that are obvious when reading both the books. Most of these aren't that difficult to spot once you read both the books.
The blatant differences:
Firstly, Atticus has changed.
In "Mockingbird," Atticus Finch, Scout and Jem's father, is an honorable lawyer and perhaps better father who somberly defends African-American Tom Robinson at Robinson's rape trial."Mockingbird's" Atticus is so renowned that the ABA Journal headlined an article "The 25 Greatest Fictional Lawyers (Who Are Not Atticus Finch)."
He tries to bear no ill will towards anybody, regardless of color. A member of a mob "is still a man." "I do my best to love everyone," he tells Scout in "Mockingbird."
So it's a shock to encounter "Watchman's" Atticus, who serves on the local citizens' council and remembers his defense of Tom Robinson -- which, in "Watchman," concludes with a not guilty verdict -- with "an instinctive distaste." He's also a former Klan member, though he went "to find out exactly what men in town were behind the masks." (The Klan is alluded to in one "Mockingbird" line as a "political organization.")
It's a challenge envisioning Gregory Peck -- who played Atticus in the 1962 "Mockingbird" movie -- playing this version of the attorney. He certainly would have given it a layer of darkness.
However, this Atticus still loves his daughter. It's the question of whether his daughter can love him that forms the crux of the book.
Secondly, Jean Louise is fully grown
"You want to grow up to be a lady, don't you?"
Her response: "Not particularly."
The pants-wearing Jean Louise Finch of "Watchman" is more ladylike than the overalls-wearing Scout, but she still doesn't fit in with the demure women of Maycomb. She dislikes the town "Coffees" and obviously prefers the freedom of New York.
"Good Lord, Aunty," she tells Atticus' sister, Alexandra, at one point. "Maycomb knows I didn't wear anything but overalls till I started having the Curse."
The coming of age of "Watchman's" Jean Louise is starker. There are passages of her as a teenager, going on a date and defending her adopted Manhattan. Gone is the wonder of young Scout.
Lastly, the timelines of the plot.
Though "Watchman" dates from about 1957, its plot -- pitting the locals' defense of the South's old ways against Jean Louise's distaste for them -- is surprisingly topical given the recent controversy over the Confederate flag.
The N-word is used with some abandon and Uncle Jack, in particular, tries to explain the region to Jean Louise as "a separate nation" that fought the Civil War "to preserve their identity."
"Watchman" also has a distinctly modern feel compared to the Depression-set "Mockingbird." The roads are now paved, there are mentions of television and the city is coping with the strivings of returning veterans. It's a far cry from the sleepy village of the'30s.
Source / further reading:
Harper Lee's 'Mockingbird' and 'Watchman' an intriguing comparison by Todd Leopold, CNN, 14 July 2015.