As you probably know if you ever configured the linux kernel yourself (possibly by make menuconfig, or if you've been with linux long enough, by make config), the current kernel configuration that would be used for build is stored in the file .config.
The options in the .config file depend on the kernel version the file was produced with. So, you can't directly use a config file from an older kernel with a newer one. But what you can do is use make oldconfig. This way, the config options that are still present in the current kernel keep their values and you are asked to configure only what options were added.
To use this, just place the old .config file into the new kernel source tree and issue make oldconfig.
Yeah, this post was inspired by the comment asking for newer ALIX 2 kernel config. A comment that I missed. Sorry. Mike. :-(