Because Ra stood at the summit of creation, Egyptian religion repeatedly fused other great gods with him rather than replacing him. The most important fusion was Amun-Ra: Amun, the hidden god of Thebes, was joined to Ra the visible sun to make a supreme national deity whose priesthood at Karnak became one of the wealthiest institutions in the ancient world. This is a characteristic Egyptian move, syncretism that adds a god's power to Ra's without either losing identity, so that a worshipper could address Ra, Amun, or Amun-Ra as the theology required.
The exception proves the rule. In the fourteenth century BCE the pharaoh Akhenaten elevated the Aten, the visible sun-disc itself, and suppressed Amun and the other gods in a short-lived move toward the worship of one solar deity. Scholars debate how far this was true monotheism, but its target was the Amun priesthood, and after Akhenaten's death the old cults were restored and his monuments defaced. Even this upheaval was fought out in the language of the sun.
Kingship, too, ran through Ra. From the Old Kingdom the pharaoh was titled Son of Ra, and the daily temple ritual re-enacted the king serving the sun-god on behalf of Egypt.