Correct me if I'm wrong. If you didn't do a "make mrproper" or "make distclean" then any compiled oject files from previous kernel compilation are still in the source tree. Now you just need to copy your config file (in /boot) to the source tree
# cp /boot/configxxxxx /usr/src/linux/.config
and make any change to the configuration. Then "make && make modules_install". I think the process will take much less time than clean compilation since it only compiles any new changes you made.