The recommended answer, as the comment suggests, is to save it as
.config in the top-level source directory, and then run
make xconfig (GUI, easier) or
make menuconfig (TUI) on a 64-bit system.
That said, to simply switch from 32-bit to 64-bit without changing anything else, a little editing at the beginning is all that's needed. Compare:
# CONFIG_64BIT is not set
# CONFIG_X86_64 is not set
# CONFIG_X86_32 is not set
- Note that
CONFIG_X86=y is not touched.