Aegis is more than the radar

Is Aegis the radar on a US warship?

No. Aegis is the integrated command-and-control system; SPY is the radar family.

Aegis is the integrated command-and-control system rather than a radar by itself. SPY is the radar family that supplies sensor data to that wider combat system. The distinction matters when comparing upgrades because sensors, weapons, software, and the command network are related but not interchangeable components. Retrofitting Aegis into an older ship such as Enterprise would demand extensive recabling and integration work rather than a simple radar replacement.