The significance of Walter's change cannot be overstated. His transformation is representative of the entire family's progress in the play (from disjointed to unified). His change also stands as a primary articulation of 1) the play's conflicts regarding dignity and identity and 2) the play's themes relating to individual possibilities, choice and empowerment, and the relationship between social class conflict and identity.
Walter initially is a sufferer, mastered by his circumstances. His only hope is to change those circumstances. When the possibility of using inheritance money to change his circumstances is eliminated, Walter immediatly slips into despair. This is a powerless man.
Walter's attitude has a significant impact on his wife, mother and sister as they struggle in their own ways to come to terms with their circumstances as well.
The prospect of a new child disturbs him. The prospect of continued poverty distresses him. The pride his mother hopes will find him looks like an unlikely outcome when his liquor store plan falls through. However, the change does come.
When his friend runs off with the money, Walter feels particularly hopeless, ironically, however, he achieves a sense of himself as an adult and leader of his family in part through this event.
Walter's transformation brings positive hope to the family, along with pride and unity. In his change, Walter has discovered that he has the power to change himself even if he does not have the power to change his circumstances.
...whether or not he achieves the American Dream, he does achieve a sense of himself as an individual with power and the ability to make choices.
This power is highly significant to the play. An assertive and dignified family has found its footing in a world where these qualities are not easily achieved. The play explores the difficulties that make this achievement so difficult, but, importantly, Walter demonstrates that this transformation and achievement is possible.
This is not a victory over circumstances, but a victory despite circumstances.
I think that the most significant element of Walter's change is that he ends up becoming a transcendent figure by the end of the drama. At the start of the drama, he is hopeless in his temporality and his contingency. He presents himself as this in his unhappiness and constant feelings of helplessness. His characterization is one that shows that him to be constantly contingent on the conditions of what are. Walter lives for the temporary happiness of small money, as well as the basic idea that somehow his life will change without any real substantive change from his end. The fact that he lives a life without any real fulfillment and that his plans for the liquor store fall through are representative of this. Walter's ability to stand up against Lindner is where Walter ends up gaining a sense of what can be as opposed to what is. His ability to stand up against Lindner's intimidation and refuse money in exchange for his dignity is where Walter shows a pivot towards what can be as opposed to what is. It is here where such transformation becomes vital to the themes of Hansberry's drama.