  A proper choice of subsystems for a system of identical particles e.g.,
bosons, is provided by second-quantized modes i.e.,creation/annihilation
operators. Here we investigate how the entanglement properties of bipartite
gaussian states of bosons change when modes are changed by means of unitary,
number conserving, Bogolioubov transformations. This set of "virtual"
bi-partitions is then finite-dimensionally parametrized and one can
quantitatively address relevant questions such as the determination of the
minimal and maximal available entanglement. In particular, we show that in the
class of bipartite gaussian states there are states which remain separable for
every possible modes redefinition, while do not exist states which remain
entangled for every possible modes redefinition
