I know that Lincoln's views changed a lot from when he was growing up to the Lincoln-Douglas debates, but how did his views change? From what?
Lincoln changed his views on a number of issues over time. For instance, he started out "pro business," and later became the "prairie populist" defending people against business interests. But the greatest change in Lincoln's philosophy came on the issue of slavery. Up to about 1850, he believed that "I do no more than oppose the extension of slavery."
That's because the Louisiana Purchase had given the free states more room for expansion than the slave states. As shown in this map, the free states could expand into the area in the dark green, while the slave states were restricted to the southern light blue area (Arkansas, Oklahoma) plus Louisiana and Missouri. These parameters were drawn by the Missouri Compromise of 1820.
But the acquisition of Texas and California, and the land in between changed that. First, the South beat back the so-called Wilmot Proviso, that would have outlawed slavery in the new territories. Then Douglas proposed the Kansas Nebraska Act that rolled "back" (northward), the Missouri Compromise line of 36' 30, meaning that a whole tier of states including Kansas, Colorado, Utah and Nevada were eligible for slavery; even California, which had elected to enter the Union as a free state. Now it seemed that slavery could not be "contained."
So during the Lincoln-Douglas debates, Lincoln gave the so-called "House Divided" Speech, which read in part: "A house divided against itself cannot stand. I believe this government cannot endure permanently half slave and half free. I do not expect the Union to be dissolved—I do not expect the house to fall—but I do expect it will cease to be divided. It will become all one thing, or all the other." By that time, he had become an "abolitionist," who wanted slavery ended altogether.