Looking at the file, it seems to consist of at least two sections catenated together.
The first section is a header that loads the compressed image into memory. The second section is compressed with either compress or bzip2, depending on if you did a “make zImage” or a “make bzimage”.
This isn't going to be a simple conversion; the “header” is different (contains a different uncompression algorithm) for compressed and uncompressed kernels.
The easier way is probably to rebuild the kernel. To do that, boot from it, cd to the kernel sources and run
Code:
make cloneconfig vmlinux
Or is there some reason why you don't want to recompile?