The OpenWRT Wiki says you can't keep existing binaries, but need to reinstall them.
In particular, any manually installed software packages you may have
installed after the initial OpenWrt installation have to be
reinstalled after an OpenWrt upgrade. That way everything will match,
e.g. the updated Linux kernel and any installed kernel modules.
Any configuration files or data files placed in locations not
specifically listed as being preserved below will also be lost in an
OpenWrt upgrade. Be sure to check any files you have added or
customized from a default OpenWrt install to back up these items
before an upgrade.
However, Tolaris gives the following command to take a list of your installed packages:
opkg list-installed | cut -f 1 -d ' '