Class mobility--moving between classes--might work in both directions in the period covered in Jane Austen's last completed novel, Persuasion. People from lower classes might move to higher classes through acquisition of money or through marriage. People from higher classes might move to lower classes through the loss of wealth, loss of friends, scandal, or criminal punishment.
In Persuasion, Lady Russell persuades Anne to reject Wentworth's early proposals of marriage, even though he and Anne truly loved each other, because Lady Russell had no expectation or hope that Wentworth could ever acquire enough money or a high enough station in life to make him worthy of Anne's family, as her father was a Baronet. In other words, Lady Russell didn't believe Wentworth could rise to a higher social class. It turns out Lady Russell was short-sighted and probably prejudiced as she didn't take into account the advantages that a career in the Royal Navy might provide a good seaman who was able to advance his rank.
When Wentworth returns to England he is a Captain and has amassed a fortune of his own; he has proven that Lady Russell was incorrect and narrow minded. His rank and wealth and higher social class now make him a very suitable match for the daughter of a Baronet. While he was gone, however, Anne suffered an opposite fate.
A woman who has no independent fortune of her own settled on her by a mother or another relative and who can depend only upon her living father's allotment of a small share of his fortune (and in Anne's case, a father's dwindling, wasted fortune) would suffer a decline in social class. If continuing to be unmarried (as Anne and her sister were), she would become dependent upon her parent(s) while they lived and then, after their deaths, upon the charity of any relative who would take her in. This was Anne's situation, and so if Wentworth hadn't returned and renewed his love for her, Anne would have gone down in social class--in fact, had gone down somewhat in class already as an unmarried woman in her late twenties.