- stay levitating at their new height?
- fall back to their previous height?
- something else?
The Levitate spell does not limit the altitude a creature can levitate at. There is only one limit specified:
One creature or object of your choice that you can see within range rises vertically [and] must remain within the spell's range.
The spell's range is measured from the caster, which is the thing being teleported. As a result, there is no conflict with being teleported to a higher altitude which would cause the Levitate spell to end or perform differently.
Further, as Dimension Door does not require concentration, it does not break concentration for Levitate. The end result is that the caster, already levitating, would teleport to a new altitude and continue to levitate there until the spell ended (or was caused to end by some other means).