The main theme of That Was Then, This Is Now is the transition from adolescence to adulthood. The novel explores how relationships and perceptions change as characters face new responsibilities and challenges, leading to personal growth and a deeper understanding of the complexities of life.

Bryon's relationships with girls in "That Was Then, This Is Now" reveal his evolution from a shallow "lady killer" to someone capable of genuine affection. Initially, he flirts and lies to girls, showing immaturity and low self-esteem. However, his relationship with Cathy marks a change, as she is not intimidated by other women and offers a contrast to his previous partners. This shift highlights Bryon's growth and his realization of the superficiality in his past relationships.

In S.E. Hinton's That Was Then, This Is Now, Mark and Bryon start as close friends and foster brothers, bonded by shared experiences and struggles. However, their relationship deteriorates as Bryon matures, influenced by his girlfriend Cathy and personal reflections on morality. Bryon strives to reform and find legitimate work, while Mark turns to drug dealing. When Bryon discovers Mark's illegal activities, he reports him to the police, leading to Mark's arrest and their relationship's collapse. Bryon learns the importance of doing the right thing despite personal loss, while Mark's actions underscore the consequences of moral indifference.

When Bryon says "that was then, this is now," he acknowledges the inevitable changes that come with growing up. He reflects on how childhood friendships and experiences have evolved as people enter new life stages, such as relationships and personal growth. Bryon understands that the past is different from the present, emphasizing the need to accept life's constant changes and the end of childhood as they transition into adulthood.

Mark's statement "That was then, this is now" signifies the permanent change in his relationship with Bryon. Despite their past as best friends, Mark's feelings towards Bryon have shifted irrevocably after Bryon reported him to the authorities. Mark uses this phrase to highlight that their bond and the good times they shared belong to the past, and he no longer considers Bryon a friend due to the betrayal.

Bryon says this after emotionally tending to Cathy. It becomes evident that she is worried about her brother and finds herself challenged by the situation regarding her brother's drug use. Bryon tries to provide emotional comfort through words and actions. Merely being there, expanding his scope of compassion and empathy for someone in need proves to be a challenging reality. It is in this in which Bryon speaks of the emotionally tiring quality of caring: "I was tired. I felt empty and drained. Nothing can wear you out like caring about people." This becomes a critical element in Bryon's transformation. He shows himself willing to submit his being to something larger.

Angela and Cathy differ significantly in personality and family influence. Cathy grows up in a loving, supportive environment that fosters her compassionate and generous nature. Her family's healthy dynamics, particularly their care for her brother M&M, contribute to her maturity and stability. Conversely, Angela is shaped by observing her brothers' negative behaviors, leading her to become manipulative and vindictive. Their contrasting home environments greatly impact their character development and life outcomes.