I hardly remember anything about the series, except that a girl meets a boy from the future, and he takes her back there. The society has become double-tiered, with the rich living on a new surface that's been built on stilts all across the world, whilst the poor still live on the ground, and have access to much sparser resources.
Body-morphing is also really easy in this world, and I remember the boy persuading her to get a slimmer and more curvaceous body. I also have a vague recollection that at least one of the books was bound back-to-back, so it had two front covers and no back cover. I read it in the early 00s/late 90s, and I don't think the books were particularly old at the time. They were definitely marketed as fantasy/sci fi light trash for teen/pre teen girls.