Front-end engineering teams routinely face a transition problem when experienced individual contributors move into technical lead positions. The shift from writing code to coordinating a team's output touches every stage of the software development lifecycle, from sprint planning and backlog grooming through code review, architecture decisions, and incident response. Despite the frequency of this transition, few formal frameworks exist to structure the handoff of responsibility in a way that protects team velocity while growing new leadership capacity. The work defines a phase-gated transition structure that maps the movement from individual contributor to technical lead across four distinct phases, each tied to specific SDLC activities and measurable gate criteria. A decision classification taxonomy organizes technical choices by reversibility and scope, linking each category to a minimum authority level in the transition sequence. Simulated evaluations across a six-sprint observation window show that structured transitions can limit team velocity loss to under 12 percent during the acting-lead phase, compared to unstructured transitions where velocity drops of 20 to 30 percent are commonly reported. The framework applies to agile front-end teams of four to ten engineers and does not depend on any particular technology stack.