You can apply your current
.config to a newer version of the kernel; they're tagged, and the make system will update it appropriately without changing what you have -- that's not a guarantee, of course; there may be some kind of incompatibility that requires a change. I can't recall noticing anything like this, however, but I usually go in short steps. You will probably be fine going from 2.6.x to 3.0 and any version of 3.x to any higher version.
However, you do have to run
make menuconfig to perform this update -- if you keep a copy of the original, run
make menuconfig, don't change anything and just save and exit, you will notice
.config has changed.
You can also run
make oldconfig, which will step you through a (possibly long) list of new choices. I'm not sure what the policy is with respect to
make menuconfig's automation of this process, but what it seems to be is that at least some new choices which are possible with your existing config are ticked, as modules where possible (the new .config is often substantially bigger).
In any case, I recommend just running
make menuconfig, again, you don't have to change anything. I've never had a problem this way, or at least not one serious enough for me to remember.
You may be interested in "Where to start with configuring, compiling and installing a custom Linux kernel?".