The series started out as the adventures of a boy with ten sisters. Eventual episodes gave the sisters and the rest of the family more focus, with numerous stories in which they're the main characters. In some episodes, Lincoln doesn't even appear at all.
In the show's premiere episode "Left in the Dark", some characters have different designs than in later episodes:
Lisa's shirt is dark green instead of chartreuse.
Lucy's hairline is raised up a little.
Clyde's hair and face are shaped differently.
Leni's dress appears with a slightly more de-saturated shade than usual.
Up until "11 Louds a Leapin'", Mr. and Mrs. Loud's faces weren't seen. However, they were hidden again in "The Old and the Restless", "Brawl in the Family" and "Making the Grade" since they were produced before "11 Louds a Leapin'."
In the episodes where Lincoln would break the fourth wall and the stories were played out from his perspective, each of his sisters' default personalities were far more exaggerated, making them seem far more one-dimensional and stereotypical.
Two early episodes show Lincoln reading comic books in his underwear, as he claims reading them with clothes on would be uncomfortable. However, later episodes show him reading comics while fully dressed with no problem (though some imply he still reads in his undies occasionally).
Rusty Spokes was introduced as one of the members of a bratty biker gang in "Hand-Me-Downer". In later episodes after this, he becomes a part of Lincoln's friendship circle along with Clyde, Liam and Zach (with Stella being added even more later).
When we first see the Loud family at dinner in "A Tale of Two Tables", there were two sets of tables, the adult table for the five eldest sisters and parents where they would eat "grown-up" food such as liver, and the kids table for the five youngest sisters and Lincoln where they would eat "kiddie" food such as chicken nuggets. Since "Fed-Up", all the family members sit at a single dining room table together and would eat whatever's on their plate since, being a big family, they can't afford to indulge picky eaters.
Clyde's Precocious Crush on Lori was a Running Gag of his character throughout Season 1 and 2. It was quietly dropped in Season 3 after Chris Savino's firing for sexual harassment, and Clyde has since seen himself falling in love with girls closer to his age, such as in "Be Stella My Heart" (Stella) and "Brave the Last Dance" (Emma and Chloe).
In early episodes, at least until after Season 2, Luna could often be seen carrying or playing with Lily in group scenes. After Season 2, it's Leni who's primarily seen with Lily doing the same activities.
Early lore for the show stated that Lisa had already finished school up through getting a PhD. The writers apparently thought this was stretching the limits of the show, as later episodes show her attending kindergarten/1st grade with other kids her age.
Lori's flatulence problem was a bit of a Running Gag throughout Seasons 1-3. It was quietly removed from her character in Season 4 onwards and hasn't been brought up or even referenced since. Of course, this could all be due to the lack of screen time she has received after Season 3.
The comic's first two issues both featured a unique table of contents page depicting the refrigerator in the Loud house's kitchen, with the names of the stories in each issue being depicted on things like shopping lists, report cards, etc. Live Life Loud! was the first issue with a simpler, more straightforward table of contents page: a solid color page, depicting one or more characters from the issue in question, with the stories all listed in order.
A few stories in early books use the early character designs from "Left in the Dark", and for example both "The Spot" and "Sam's Pick" use the original Lucy design with her bangs leaving a small patch of skin over her nose uncovered.
The first book was written at the time when the Loud parents were still The Faceless. As a result, they don't appear for most of the book, with the only exceptions being Lynn Sr. appearing in "Here Comes the Airplane", with his face being kept out of frame as it happened during the show's first season.
The first issue, There Will Be Chaos, has "Lost Control", a choose-your-own-adventure-style story involving Lincoln searching for a lost TV remote (which actually started off as a series of posts on the franchise's Instagram account). This concept of a miniature gamebook has never been tried again since.
The first three issues end with previews not of the next book in the series, but rather of adaptations of other Nicktoons that were printed on Nickelodeon Pandemoniumnote A short-lived series of books by Papercutz that presented in digest format select stories from all their Nickelodeon comic books at the time - The Loud House, Pig Goat Banana Cricket, Sanjay and Craig, Breadwinners and Harvey Beaks. The first two issues feature previews of Pig Goat Banana Cricket comics, while the third has a Harvey Beaks one.
