While there are different themes in That Was Then, This Is Now, I think that one of the strongest is maturation.
Byron's coming of age drives the story. At the start of the story, Byron and Mark share the same values. Their priorities are similar in terms of enjoying life in the moment. However, their paths begin to separate as they get older. The need to help their mother is one area where this divergence is evident. Byron struggles to get a job and begins to think about life differently. His maturity is seen in how he gets a “haircut, clean clothes, and a really big change in attitude.” Byron's change and Mark's lack of it becomes a critical part of the story, something that Byron himself notes: “I was changing and [Mark] wasn’t.”
As a result of having to make different choices than Mark, Byron begins to distance himself from who he once was. This is enhanced when he dates Cathy and says “I had quit thinking only about myself.” Byron develops as a person in how he takes a larger view towards life, something that Mark fails to do. Byron matures in standing for something larger than himself, eventually leading him to turning Mark into the authorities. Byron's growth is the difference between both. Mark cannot believe that Byron would call the authorities to turn him in. The reason Mark is shocked at Byron's actions is because of the change that Byron has experienced, clearly showing how Byron's maturation impacts both of them.
The novel explores how people change as they come of age. As individuals come of age, the novel shows how they begin to think about things larger than themselves. Maturation is shown to define people and underscore their differences with others. The novel's title reflects how when an individual "grows up," it impacts their relationships with others.