From On Christian Doctrine:
60. Moreover, if those who are called philosophers, and especially the Platonists, have said anything that is true and in harmony with our faith, we are not only not to shrink from it, but to claim it for our own use from those who have unlawful possession of it.
God is Open 
Perhaps what Augustine has in mind is the relationship between a steward/guardian and a legitimate heir. The philosophers possessed true notions of theology, not as rightful owner, but as stewards who are in charge of the estate of knowledge, until the arrival of the legitimate heirs.
Augustine had an interesting thought process on the Platonists. Augustine states (hypothetically) that if he were a Christian, but then came across the work of the Platonists, then he would have converted to Platonism. He adds that it was because he was first a Platonist, that he was able to interpret the Bible in light of Platonism, and then accept Christianity.
So while Augustine discounts the Platonists as a little less than Christian, he embraces them over the theology of most Christians who take the Bible in a more literal light. The Platonists were the torch bearers, Augustine shows solidarity with them on multiple occasions, but Augustine adds charity and believes he has found a better form of theology.
While there are many commonalities between Platonism and Christianity, there are profound differences as well, among others, Charity, as you pointed out. Since Augustine believed he found a better theology in Christianity, I don’t think he would’ve converted to Platonism, for that would be regress, not progress. :)
I am quoting Augustine where Augustine says exactly what I attribute to him:
“…or had I first been moulded in Your Holy Scriptures, and had Thou, in the familiar use of them, grown sweet unto me, and had I afterwards fallen upon those volumes [the books of the Platonists], they might perhaps have withdrawn me from the solid ground of piety; or, had I stood firm in that wholesome disposition which I had thence imbibed, I might have thought that it could have been attained by the study of those books alone.”
You should see this article:
https://godisopen.com/2015/01/15/apologetics-thursday-greek-influences-in-the-church/
The passage immediately preceding the one you quoted above, and the whole context of Book VII of the Confessions, makes it clear what Augustine believes is the difference between Platonism and Christianity.
“I might discern and distinguish what a difference there is between presumption and confession,— between those who saw whither they were to go, yet saw not the way, and the way which leads not only to behold but to inhabit the blessed country.”
It’s not merely a difference of interpretation, but the difference between perception/speculation on one side, and reality/true knowledge on the other. To borrow a Biblical imagery, there is a great gulf fixed between Platonism and Christianity.
If Augustine had come across Platonism after having been familiar with Christianity, it might have withdrawn him “from the solid ground of piety”, because the former does present an attractive image of the reality with which a Christian is familiar, as images projected on the cave wall might deceive people into thinking they are real; however, since he started with Platonism, he was well aware of its limitations, and knew that it was but a shadow, and therefore did not mistake the shadow for the real thing, i.e., the Truth to be found in Christianity alone.