The legacy agent constructors in the langchain package, including initialize_agent and the AgentExecutor built from an AgentType enum, are deprecated. The maintained replacement is create_react_agent from langgraph.prebuilt, which builds a graph rather than a chain.
The migration is not a drop-in rename. The graph returns a state dict containing the full message list rather than an output string, so callers that read a single output key need updating. Tools are passed directly instead of through a toolkit, and the prompt is supplied as a system message or a modifier rather than through the old agent-specific prompt templates.
What you gain is where the difference shows up in production: persistence through a checkpointer, so a conversation can be resumed by thread id, interruption points for human approval before a tool runs, and streaming of intermediate steps. Those were the features people wrote fragile callback handlers to approximate under the old executor.