The question says it all. When was the first mention of the Golden path in the books and when it was mentioned did it mean what it meant at the end of the series? Note that I do not want an answer from anything other than the original 6 novels.
The first mention of the Golden path was at the beginning of Children of Dune when Leto and Ghanima call up the memories of their parents:
Her voice fell into the desert silence which even a whisper might despoil, and Leto felt himself sinking, sinking -- becoming the father whose memories spread like an overlayer in the genes of his immediate past.
For this brief space, I must be Paul, he told himself. This is not Ghani beside me; it is my beloved Chani, whose wise counsel has saved us both many a time.
For her part, Ghanima had slipped into the persona-memory of her mother with frightening ease, as she had known she would. How much easier this was for the female -- and how much more dangerous.
In a voice turned suddenly husky, Ghanima said: "Look there, beloved!" First Moon had risen and, against its cold light, they saw an arc of orange fire falling upward into space. The transport which had brought the Lady Jessica, laden now with spice, was returning to its mother-cluster in orbit.
The keenest of remembrances ran through Leto then, bringing memories like bright bell-sounds. For a flickering instant he was another Leto -- Jessica's Duke. Necessity pushed those memories aside, but not before he felt the piercing of the love and the pain.
I must be Paul, he reminded himself.
The transformation came over him with a frightening duality, as though Leto were a dark screen against which his father was projected. He felt both his own flesh and his father's, and the flickering differences threatened to overcome him.
"Help me, father," he whispered.
The flickering disturbance passed and now there was another imprint upon his awareness, while his own identity as Leto stood at one side as an observer.
"My last vision has not yet come to pass," he said, and the voice was Paul's. He turned to Ghanima. "You know what I saw."
She touched his cheek with her right hand. "Did you walk into the desert to die, beloved? Is that what you did?"
"It may be that I did, but that vision... Would that not be reason enough to stay alive?"
"But blind?" she asked.
"Where could you go?"
He took a deep, shuddering breath. "Jacurutu."
"Beloved!" Tears began flowing down her cheeks.
"Muad'Dib, the hero, must be destroyed utterly," he said. "Otherwise this child cannot bring us back from chaos."
"The Golden Path," she said. "It is not a good vision."
"It's the only possible vision."
Children of Dune
It is not explained at this point, or indeed in any detail through the entire book. We find out that Leto encasing himself in sandtrout is a step on the Golden Path, but the most we learn about it is a few second-hand hints dropped by Stilgar:
"Last night he told me about this Golden Path," Stilgar blurted. "I don't like it!"
"That's odd," Ghanima said, glancing at her grandmother. "Most of the Empire will welcome it."
"Destruction of us all," Stilgar muttered.
"But everyone longs for the Golden Age," Ghanima said. "Isn't that so, grandmother?"
"Everyone," Jessica agreed.
"They long for the Pharaonic Empire which Leto will give them," Ghanima said. "They long for a rich peace with abundant harvests, plentiful trade, a leveling of all except the Golden Ruler."
Children of Dune
Since almost no details are revealed here, it's almost impossible to say if the meaning of it changed over the course of the series. I would argue that the meaning didn't change since the details were only ever revealed when they became relevant to the choices and actions of the moment.