The original command to love thy neighbor as thyself was a command given to Israel in Leviticus. It is specifically a covenantal command in the Hebrew Bible. The law bears this out, as Israel was to not covet their neighbor's possessions, and yet were to take all of the possessions of the Canaanites. Were the Canaanites not Israel's physical neighbors? Is taking one's land, killing him, killing his family loving him as one loves himself? We see a massive conflict when moderns interpret the neighbor to be everyone, since God specifically commands that one love neighbor as self, but then commands that Israel drive out the Canaanite and utterly destroy him.
There is no conflict in the original idea, however, because neighbor, as stated before, refers to a member of the covenant community in the context of the Leviticus passage, which is the command upon which the New Testament bases its directives concerning the neighbor.
All of this is ignored by a modern religious culture that has been steeped in the "fatherhood of God, brotherhood of man" denial of the exclusivity of the kingdom and its physical resources.
What is done, however, is the typical neo-Marcionite move that argues that God only considered covenant members to be "neighbors" in the Old Testament, but now Jesus has made non-covenant members neighbors in the New Testament. This is mainly argued through the parable of the Good Samaritan and the idea that Jesus changes the law in some way.
The neighbor idea, however, is never expanded to include non-covenant members. Instead, the covenant always included ethnic and non-ethnic Israelites, who are worshipers of YHWH (the latter often described as the "sojourner" or "resident alien," which aren't the best translations). The New Testament simply brings this out all the more, and of course, the Gospels make it clear that the Jews are not the only covenant members, but even Gentiles who repent and evidence their covenant status by how they treat other covenant members should be considered such as well. Again, nothing different here.
So the claim that it has somehow changed is odd. The extension of the gospel is one that reaches to all who repent, Jew or Gentile. The covenant does not stretch to unbelievers. It never has and it never will. So the neighbor command in Leviticus remains the same command in the New Testament. You are to love your believer as yourself. And this is why the New Testament continually exhorts Christians to take care of "one another," to meet the needs of the brother who represents the invisible God, the least of Christ's brothers who stand in his place upon the earth.
The gospel is exclusive. Our religious culture doesn't like that. It is no surprise that it doesn't like exclusiveness of the kingdom and its resources either. But any argument that says God's covenant blessings are exclusive in the Old Testament, but inclusive in the New, is simply arguing for a Bible of two religions, one of the OT and one of Jesus. Marcion would be proud, but it doesn't endure the weight of scrutiny. The only reason why so many put up a fuss about it is not due to the evidence we have, but because one has a theological horse in the race; but that's not scholarship. It's apologetics, and apologetics that ignore the texts involved is always eisegetical and not worthy of our time.