The package tools support in-place updating using pkg_add -u.
For instance, to update all your packages, make sure PKG_PATH
is pointing to the 4.2 packages directory on your CD or nearest
FTP mirror, and use something like

    # pkg_add -ui -F update -F updatedepends

where the -u indicates update mode, and -i specifies interactive
mode, so pkg_add will prompt you for input when it encounters some
ambiguity. Read the pkg_add(1) manual page and the package management
chapter of the FAQ for more information.