  We illustrate how form-invariance transformations can be used for
constructing phantom cosmologies from standard scalar field universes. First,
we discuss how to relate two flat Friedmann-Robertson-Walker cosmologies with
different barotropic indexes $\gamma$ and $\bar \gamma$. Then, we consider the
particular case $\bar \gamma=-\gamma$, and we show that if the matter content
is interpreted in terms of self-interacting scalar fields, then the
corresponding transformation provides the link between a standard and a phantom
cosmology. After that, we illustrate the method by considering models with
exponential potentials. Finally, we also show that the mentioned duality
persists even if the typical braneworld modifications to the Friedmann equation
are considered.
