This is my first post here, hopefully this fits the community's standards. I'm not looking for the mythology importance of this switch, or religious practices involved (though if those are available, awesome!), but I'd like to find out how the civilization reacted to the change, and what events were necessary for it to happen. If anyone knows any good articles or other sources on the subject, I'd greatly appreciate those as well!
I have to start by disagreeing with your premise. It didn't go: Amun → Amun-Re → Re. Also, minor point, but his name is Re. "Ra" is the Egyptological pronunciation of 𓂋𓂝𓇳 and is used for convenience to represent the consonants written in Hieroglyphs. It has no meaningful relationship to any phonetic reality. Re is the Egyptian word for "sun". Not just a deified Sun god creature, but the regular old sun as well.
Re came to prominence with the Heliopolitan Solar Cult during the Old Kingdom (ca 2686 – ca 2160 BCE^1). That's when we start seeing sun temples with weird fat obelisks popping up. It's also where Heliopolis gets its (Greek) name: Ήλιού πολις = "sun's city". Here's a good summary of that. In the Pyramid Texts, written during this time period, Re is ubiquitous while Amun is rare (at a ratio of around 100:1 by my quick estimation).^2 Re continues to be an important deity from that time forward, especially in funerary literature, where his passage through the underworld and dualistic relationship with Osiris is a major theme.^3
Amun is different. He began as the patron god of the city of Thebes, and he didn't really rise to prominence in the whole of Egypt until the Second Intermediate Period (ca 1650 – ca 1550 BCE), when he is assigned the role of patron deity of the attack on Avaris.^3 Long story short: Local Theban rulers ousted Semitic rulers in the Delta and reunified Egypt, producing the New Kingdom (ca 1550 – 1069 BCE). Amun expanded from his Theban origins to become patron deity over all of Egypt. In addition to his temples at Thebes, other temples sprang up from the Delta to Nubia. When the Nubians began invading Egypt, eventually leading to the 25th Dynasty (744 – 656 BCE), they cited Amun as their patron deity and stopped in Thebes to worship him on their way down the Nile.^5
Amun really comes into his own during the Third Intermediate Period (1069 – 664 BCE), when Egypt was a loose theocratic confederation with Amun as its de jure pharaoh.^6 This is a whole big topic, but you can put the pieces together from a variety of sources. The Report of Wenamun has its titular character running around the Mediterranean with a golden statue of Amun demanding that people do as he asks for the god's sake. It's pretty remarkable and more than a little comical. Most remarkable of all is that rulers in Thebes and the Delta split control of the country amicably, agreeing that Amun was the true king and they were his servants. The Cult of Amun is what allowed Egypt to function efficiently without a centralized government for 500 years.^7
I've had many students ask, "What exactly is Amun the god of?" because we naturally expect that he should have some defining feature. The answer I always give them is to think of Captain America. What is he the god of? Well, he's somehow a corn-fed farm boy with blue eyes and blond hair ... from Brooklyn, so he's like a personification of the United State's dominant view of itself. That's Amun. He's Mister Egypt.
Syncretism is another massive topic that we could pull out a pile of books on, but basically, when different deities have similar roles and attributes, it's natural to imagine that they are manifestations of the same underlying divine power. It's also theologically convenient in a number of ways, which depend on the circumstance. In this case, Re is the biggest, most important deity in all of Egypt, but so is Amun. How can they both be the most everything? Well, they're really two versions of one guy.
That's super brief, but you can always read more about syncretism as a major force in Egyptian religious thought. I recommend anything by Assmann, especially IIRC: The Search for God in Ancient Egypt.
