Marvel did a number of variations on their superhero lines, involving using varied time periods. For example, they placed X-men during the time of the Civil War.
I got to thinking what that would be like for Animorphs. Lets say the Yeerk Invasion began a lot sooner than the late 20th century. What would the characters be like?
For my money it would play out something like this.
Jake's Dad would be a Union Doctor drafted to war to help heal and treat injured Civil War veterans. Jake himself would be much the same but his relationship with Marco, who has hispanic heritage, might be extremely strained because of the social climate of the time.
Cassie's parents would likely be recently freed slaves, who came into posession of the farm. Her father could still be a veterinarian of sorts, perhaps having been educated by his former master. Her relationship with Jake would be heavily frowned upon in this time, especially by his older brother, who has taken a "strange ailment", though Jake can't seem to convince his family. So everytime Cassie's father comes over to take a look at their dog Homer, he and Cassie sneak off to have a snuggle.
Rachel's father is a Confederate sympathizer and still living in the south. A position that her mother, a strong willed woman of the North furiously detests. Because of this, Rachel is equally staunch on the matter of the Civil War and while not exactly friends with Cassie, she supports her relationship with Jake and defends all of the black people she knows. She believes woman should vote and is quite outspoken about a number of topics that annoy alot of the adult men in her life.
Tobias would be a bit more like Huck Finn. More of a survivor with his head in the clouds, he would have likely bolted from his aunt and uncle long ago.
Through strange coincidences, these five children encounter Elfangor. But with the curiosity of children from any time period, they do not flea the spaceship and much the same manner, Elfangor tells them of "demons" known as the Yeerks. He then gives them the power to morph by acquiring the DNA (Which the children come to call "Essence") of animals to use against the Yeerks.
This is just my concept.
Any other ideas? Don't limit yourself to American history. What other alternate possibilities can you think of?