  We devise a method to obtain a phantom version of FRW k-essence cosmologies
with homogeneous k-fields by applying form-invariance transformations. It can
be seen that the transformation performs the maps $H\toH$ and
$\rho+p\to-(\rho+p)$, which in turn give $\gamma\to\gamma$ and $a\to a^{-1}$.
The discussion is presented in a general setup, valid for FRW k-essence
cosmologies, and then we discuss power-law solutions for illustration purposes.
First, we deal with models such that the gradient of the k-field is not
constant, which include standard and generalized tachyon cosmologies. We
concentrate on the usual tachyon and show the phantom symmetry involves a
change in the potential and that it generates an extended super-accelerated
tachyon field. Then, we turn our attention to models for which the time
derivative of the k-field is not constant, and we show the transformation can
be implemented without changing the potential at all.
