Although today the gospels are "carved in stone" this was very much not the case back then, and the texts were subject to frequent modification. The Passion Narrative was a text that developed over time, and much of it was devised from scripture rather than history.
Why think it changed?We do not know what the original passion narrative was, let alone its version history, so why suppose it changed? We can look at the way other texts have changed. The second half of the last chapter of Mark is absent in the earliest manuscripts. The verses that are in the Bible today were a later addition - and some Bibles even acknowledge this:
This Bible page notes an additional verse in some manuscripts.
It could be argued that the authors of Matthew and Luke were not writing new gospels, but were independently updating the one gospel. Matthew is really Mark 2.0. Or rather Mark is Passion Narrative 2.0, and what we see in the Bible is 2.1, whilst Matthew is 3.0.
The narrative was not scripture to the Christians of the time, and so would not have been considered inviolate. If new information came to light, whether from a new witness, personal revelation or mere rumour, it was considered appropriate to update the record, to make it more complete.
Why think it came from scripture?After Jesus' arrest, the disciples fled Jerusalem, so there was no one around to actually see what happened. This is alluded to in Mark (see also Peter and John 21):
Mark 14:27 “You will all fall away,” Jesus told them, “for it is written:However, if Jesus was the messiah, it would have seemed reasonable to use scripture to learn what had happened. The practice of Midrash was well established, and would have seemed very appropriate.
“‘I will strike the shepherd,
and the sheep will be scattered.’[d]
28 But after I have risen, I will go ahead of you into Galilee.”
Jesus' last words come from Psalm 22:1, whilst his last drink comes from Psalm 69:21. It was usual for crucifixion victims to have their legs broken, but Jesus was excused because of Psalm 34:20. Jesus was resurrected after three days from Jonah in the whale. Why did Jesus say nothing to defend himself when accused? Isaiah 53:7. The soldiers gambling for Jesus clothing comes from Psalm 22:18, and putting a spear in his side comes from Zechariah 12:10.
Many of these details only appear in later accounts (the last two are from John for example). Clearly someone later read the text and realised that of course that must have happened to Jesus - look it says so in scripture. And so that gets added to the narrative.
How much came from scripture? We cannot tell. It is likely the disciples were familiar with Roman practices and that may well have informed the narrative. Perhaps Joseph of Arimathea was well known for being the guy who would ask the Romans to take the body down from the cross - something that he would likely do every time a Jew was crucified, maybe several times a month (assuming that even happened, of course).
The Empty TombThe early Christians did not care where Jesus was buried; Jesus had a new body, who cared what happened to the old? Furthermore, they had not been in Jerusalem for the crucifixion, so had no idea were the body was.Nevertheless, somehow the idea of an Empty Tomb appeared.
However, it is very significant that the Empty Tomb is absent from Paul's creed in 1 Corinthians 15:3-6. It had clearly not been invented at that point. It is also worth noting that the disciples make no mention of it whilst preaching, as described in Acts.
There is some evidence it was in the pre-Markan passion narrative, based on the idea that Peter and John seem to draw on that source, and both include the Empty Tomb, however John is certainly later, and could have used both the passion narrative and Mark (and the other gospels too) as sources, and Peter was heavily redacted later, so may have had the empty tomb added.
While plenty of scholars believe it did not include the Empty Tomb, I tend towards thinking the Empty Tomb was in the passion narrative, but added relatively late.
My feeling is it was invented between Paul and Mark.
More on the passion narrative here: