Xehanort contradicted himself
he said oh yeah u cant change the future blablablala
sure is that why Ansem SoD is the one who convinced young xehanort to go to darkness? without ansem sod = the future is different
No, he said you can't have a huge effect on the future, not that you couldn't change it. The most you can really do is talk to people, which he did, he talked to himself.
Who knows what was said other than telling his past self what was to come. He said he didn't know that was himself that had come to him that day, so it's obvious he didn't tell his younger self his plans in depth or anything about himself really.
All he probably did was explain a bit about darkness in his usual cryptic way and how to go about exploring the world and why darkness is so important or something to that effect.