[CentOS] Kernel version in CentOS 7 pxeboot images

I have a number of machines which PXE boot using the images provided by 
CentOS here:

http://mirror.centos.org/centos/7/os/x86_64/images/pxeboot/

That gives the machines kernel 3.10.0-327.el7.x86_64.  I would like to 
move to a newer kernel as I'm seeing a serious I/O problem which I 
suspect is a kernel bug.  I thought that would be as easy as grabbing 
the files from here:

http://mirror.centos.org/centos/7.2.1511/os/x86_64/images/pxeboot/

... but that's not the case, those are the same exact files as the 
earlier ones.  These machines always PXE boot, they have no OS installed 
to their local disk.  Instead their root filesystem is on an NFS 
server.  So this is not PXE+kickstart or other provisioning system, just 
PXE then mount an NFS export for its root filesystem.

So, how would one go about creating a new vmlinuz and initrd.img the 
same way that the ones shown above were built, except with a newer 
kernel?  Also, what is upgrade.img?

-- 
Jeff White
HPC Systems Engineer
Information Technology Services - WSU