Is this saying anything different than what it's saying in the NIV translation? No, not at all. In each translation, Jesus is saying that John the Baptist was the Elijah to come. There's a reason why Jesus said "if ye will receive it, this is Elias, which was for to come.". He knew that peope like you would have trouble accepting that. But, we should always accept what Jesus teach
Matthew 17:10-12 KJV
10) And his disciples asked him, saying, Why then say the scribes that Elias must first come?
11) And Jesus answered and said unto them,
Elias truly shall first come, and restore all things.
12) But I say unto you, That Elias is come already, and they knew him not, but have done unto him whatsoever they listed. Likewise shall also the Son of man suffer of them.
Jesus makes two statements here, one is that Elijah shall come first and restore all things, the other that Elijah has come, and they didn't know him, and did to him what they wanted. That is, they killed him.
So Elijah shall come, and already has come.
A key point is that Elijah shall fulfill his ministry, and Jesus acknowledges in this passage that this still has a future fulfillment.
NIV:
10 The disciples asked him, “Why then do the teachers of the law say that Elijah must come first?”
11 Jesus replied,
“To be sure, Elijah comes and will restore all things.
12 But I tell you, Elijah has already come, and they did not recognize him, but have done to him everything they wished. In the same way the Son of Man is going to suffer at their hands.”
NASB
10 And His disciples asked Him, “Why then do the scribes say that Elijah must come first?”
11 And He answered and said,
“Elijah is coming and will restore all things;
12 but I say to you that Elijah already came, and they did not recognize him, but did to him whatever they wanted. So also the Son of Man is going to suffer at their hands.”
YLT
10 And his disciples questioned him, saying, `Why then do the scribes say that Elijah it behoveth to come first?'
11 And Jesus answering said to them, `
Elijah doth indeed come first, and shall restore all things,
12 and I say to you -- Elijah did already come, and they did not know him, but did with him whatever they would, so also the Son of Man is about to suffer by them.'
Each of these shows "will restore" to be future, again, even though John the Baptist is dead at that time.
How do you account for that?
They killed him, and the people were not prepared to receive him. Yet Malachi and Jesus both prophesied the successful completion of his mission. How do you account for that?
Elijah came, and they killed him, so he will be here again, and will prepare a people.
Jesus came, and they killed him, so he will be here again, and will reign over His people.
Much love!