Just taking what Jesus says at face value here, yes, it does seem that he is implying that he was with the Father in heaven before becoming incarnate. The way you could confirm this in your own mind is to read the Gospel of John in one sitting at least through chapter 17. On the day following, read it again. Then read it again. Then read it again. Then read it again as many times as you can take it :-) and see if there are any other statements that Jesus makes leading up to this verse in
John 17 that would make you think in v.5 that he is in fact saying he would be returning to the place of glory where he had actually been with the Father before the Father sent him from heaven to earth.
Or you could begin with chapter 6.22-62. Here Jesus declares that he is the Bread of Life who has come down from heaven. You'll note throughout this passage that he contrasts himself with the Father (e.g. v. 27) and several times says the Father gave him or (v. 32) sent him (v. 39) and that he has come down from heaven (v. 33).
Just taking Jesus at face value, he was in heaven with the Father before coming down to earth. He says he even saw the Father before the Father sent him (v. 46).
I think the critical verses to pay attention to are v. 38 and v. 62:
"38 For
I have come down from heaven, not to do My own will, but the will of Him who sent Me."
"62 What then if you should see
the Son of Man ascend where He was before?"
Note carefully what he says in v. 38. The image is of two in heaven both with the capability of willing something. Being the Son, he accepted the Father's will.
The commissioning and yielding would have had to occur in heaven for Jesus to then say that he came down to do the Father's will.
In v. 62, he clearly says that he, the Son of Man, would ultimately go back to heaven where he had been before--
not where he had been before "as God" but specifically as the Son.
Compare this with 13.3: "Jesus, knowing that the Father had given all things into His hands,
and that He had come from God and was going to God."
After dwelling on this for a while, you may want to reflect on John 16.16-30. What has struck me about this passage is Jesus begins by telling his disciples that he is intending to speak plainly about the Father using no figurative language (v. 25). Here's his plain speaking in vv. 28-30:
"'I came forth from the Father and have come into the world. Again, I leave the world and go to the Father.' 29 His disciples said to Him, 'See, now You are speaking plainly, and using no figure of speech! 30 Now we are sure that You know all things, and have no need that anyone should question You. By this we believe that You came forth from God.'”
Jesus is speaking matter-of-factly, and so we are to take His words at face value. If we interpret Jesus’s words in any way but the most literal, matter-of-fact way, we directly go against how Jesus characterizes His own words here.
When I read all these passages at face value (and there is more in the rest of John that you will note yourself should you decide to read at least
John 1-17 straight through in one sitting several times), I don't see how Jesus is not asserting that he did in fact exist with the Father before descending to earth and becoming incarnate and having done the Father's will he would go back to the Father in heaven.
Of course, right after chapter 16 where Jesus speaks plainly about coming from the Father and going back to the Father is the verse you mention . . . :-)
I have been Oneness for a couple of decades but am currently really struggling with the Gospel of John and how it does not appear to easily fit into a Oneness framework. In the event you are interested, I started a tread about this in the Debate Room in the folder on the Godhead.
My soul followeth hard after thee....Love one another with a pure heart fervently. 
Linear Mode
