A new kernel version needs to be compiled to use a new feature. If the old kernel configuration file is
available, which make target will only update the configuration file for the new kernel version?
(Please enter only the make target.)

Answer: oldconfig

Explanation:
See http://www.gentoo.org/doc/en/kernel-upgrade.xml#doc_chap10
To reuse your old .config, you simply need to copy it over and then run make oldconfig.