Pride and Prejudice famously begins with the lines,
"It is a truth universally acknowledged, that a single man in possession of a good fortune must be in want of a wife."
Similarly, Austen concludes her novel with comments about Mr. Darcy and Elizabeth's gratitude to the Gardiners for being the "means of uniting them." In between the opening and closing lines, Austen satirizes her era's emphasis on marriages of convenience and young women's dependence on making a "good match."
In regards to the significance of marriage changing, it depends on the character. For example, marriage for Mr. Collins is significant all along because he wants to please his patroness with a suitable match. He needs a submissive, almost subservient wife, and his view of marriage's importance does not waiver. Likewise, Charlotte and two of Elizabeth's younger sisters place great importance upon marriage as a means to escape their country home and/or for financial comfort.
Darcy's and Elizabeth's perceptions of marriage are the only ones that truly change. For Darcy, a successful marriage is one which includes a respectable, intelligent, socially mature young lady. However, when he first proposes to Elizabeth, he confesses that he cannot control his feelings for her and has essentially changed in his view of marriage. Elizabeth, on the other hand, at first seems to have no desire to marry. She berates Charlotte for agreeing to marry Mr. Collins and angrily rejects Mr. Darcy's first proposal. By Chapter 35, she appears to have given up on marrying. However, after she learns the truth about Darcy and sees his true nature, she realizes that she longs for the companionship and intellectual challenge that a marriage to someone like Darcy can provide. Ironically, her marriage proves to be more similar to Charlotte's than she would like--she has financial comfort and has escaped her raucous family home.