I've got an old box running CentOS 6.2 and Windows 7. Without going into
details, this box is vital and I use it every day. Finally I decided
that I had to bite the bullet and upgrade the linux system, and I
decided to go for chimaera.

Built a new box from scratch and cloned all the disks, using dd, to
fresh HDDs (there are several big data disks in the box). Made another
clone of the first disk just for safety's sake, then installed chimaera
on free space on the first disk - successful; chimaera and windows 7
both booted fine. But CentOS 6.2 wouldn't boot - sometimes automatic
reboot, sometimes blank screen and hung box.

Switched back to the latest clone disk, which fortunately booted
successfully, made a fresh clone of the working disk, then tried again:
this time, installed beowulf. Install was successful - and this time
devuan, windows 7 and CentOS 6.2 all booted successfully.

Took another safety clone of the first disk (I'm beginning to wonder if
I've exhausted the world's stock of 2T HDDs) and then upgraded beowulf
to chimaera. Upgrade successful. Again, the CentOS 6.2 system wouldn't
boot.

CentOS 6.2 uses kernel 2.6 - it's possible to upgrade to a later one,
but this is frowned upon. I suppose it's based on RedHat and
derivatives' policy of setting a base version per distro and then
retrofitting updates. (I did once try upgrading to a v4 kernel, and the
system became completely unstable.)

Removed the primary disk, put in the clone with beowulf installed, and
verified that all was still working. Then put the disk with chimaera in
another box with identical hardware, and started digging into the
problem. Grub on chimaera = 2.04-20; on beowulf = 2.02+dfsg1-20
+deb10u4. Booted into chimaera and downloaded the packages for the
beowulf grub release (grub2, grub2-common, grub-common, grub-pc, and
grub-pc-bin), and used them to downgrade grub on the chimaera system -
successful. Rebooted - CentOS 6.2 now boots. Tried going into the grub
command line environment on each box, and using the "linux" command to
load the 2.6 kernel image: result was in grub 2.02, it works fine, and
in 2.04, the box reboots at that point.

So current conclusion is that something has happened between grub 2.02
and 2.04 which prevents the latter from loading linux v2 kernels. The
challenge now is to find out what, and if it's possible to work around
it in grub 2.04. (I should say that I originally assumed that the
problem was down to moving a disk (or a clone of it) from a non-UEFI
environment to a UEFI one - but setting everything in the firmware to
"legacy only" didn't have any effect.)

Just wondered if anyone had any thoughts and comments (other than why
the hell am I still running CentOS 6.2 on this box), before I start
rummaging through the grub changelogs. Apologies for the length of this
and also if I've missed something obvious. The above is a heavily
boiled-down summary of about a fortnight of stress and lost sleep.