  Every commutator preserving bijection of the unipotent radical $Up(2n, R)$ of
the Borel subgroup of the classical symplectic group of rank at least 4 over a
field $F$ such that $6F=F$ is shown to be the composition of a standard
automorphism of $Up(2n,R)$ and a central map. The latter is a bijection which
acts as the right multiplication by a matrix from the center of $Up(2n,F)$.
