The changes in the meaning of the Scarlet Letter depict the changes in Hester's character and society's perception of her. At first, the letter stands for adultery. Although she braids the letter and adorns it with gold threads, she is still seen as an adulteress by the community and forced to live on the outskirts of the town. However, she helps make clothes for the poor, serves a a midwife and listens to trouble people's problems. Even her clothing becomes more conservative. Thus the "A" begins to stand for "Able". At the end of her life, when she returns to Boston and continues her good deed, many people have forgotten what the "A" originally stood for. When Hester dies, the meaning of the letter has changed to "angel", because of the way she is now perceived in Boston.
In the beginning, Hawthorne goes into great detail to describe the elaborate, intense, beautiful letter A that Hester makes. This type of ornate decor was highly unusual, even looked down upon, in the Puritan community where she lived. This relates to Hester herself; she was beautiful, intense, and stood out in her community-and even did things that were looked down upon.
Later however, Hawthorne describes how the letter A comes to symbolize something different to the community; through Hester's unceasing good works, she becomes a symbol of good, of reverence to many. Likewise, the A's bright colors and ornate stitchery are seen as a symbol of bright goodness and angelic tokens of kindness.
From beginning to end, the A symbolizes Hester's progression-from rejected but beautiful outcast, to a respected figure of mercy.
I also think that the elaborate and decorative letter symbolizes Hester's ambivalence toward what the letter stands for. Hester does not feel the same personal guilt that Dimmesdale does. Her famous line, "What we did had a consecration of its own" indicates that she had a much different attitude toward her "failure" than Dimmesdale did; he was unable to live with what he did because it violated his sense of who he was. But Hester realized that her punishment was "deserved" according to the standards of her society, so she accepted their judgment. She wore the "A" they wanted her to wear, but she decorated it to reflect her attitude toward what she and Arthur had done.