(and it is kernel_oldconfig . Don't forget _ )
Basically the config routine goes through the existing config file (probably one you copied from config-4.4), and presents you with new options from 4.9 and usually offers a default answer. New options are marked with "(NEW)". kernel_oldconfig jumps over the already known options.
You should usually take the default, but in cases where the option has relevancy to the router functionality, you should make the correct selection... (at the first stage, just answer the default)
E,g, for ipq806x there are new drivers in 4.9 for new chips. Do I want them? Likely not.
perus@ub1610:/Openwrt/r7800$ make kernel_oldconfig
make: Entering directory '/Openwrt/r7800/target/linux'
make: Entering directory '/Openwrt/r7800/target/linux/ipq806x'
net/sched/Kconfig:44: warning: menuconfig statement without prompt
* Restart config...
* Qualcomm Support
Qualcomm Support (ARCH_QCOM) [Y/n/?] y
Enable support for MSM8X60 (ARCH_MSM8X60) [Y/n] y
Enable support for MSM8960 (ARCH_MSM8960) [Y/n] y
Enable support for MSM8974 (ARCH_MSM8974) [Y/n] y
Enable support for MDM9615 (ARCH_MDM9615) [N/y] (NEW)
Kernel version bump from 4.4 to 4.9 is not an easy jump. Quite much depends what has happened for that target/chipset in Linux.