Do I need Microsoft 365 E5 to start replacing perimeter-era security controls?

Not necessarily. For many organizations, Microsoft 365 E3 is enough to begin with baseline identity, device, and initial data controls, including Conditional Access foundations, Intune-based compliance, and early-stage policy enforcement. Microsoft 365 E5 becomes more attractive when the roadmap requires stronger identity protection, deeper detection, richer visibility, and broader Zero Trust coverage across the environment. In other words, E5 is often the cleaner long-term path, but replacing perimeter-era trust usually starts with architecture and rollout discipline, not with licensing alone.