[CentOS] upgrading 7.5 ==> 7.6

hI ALL!

There have been a large enough number of people posting here about difficulties when upgrading from 7. to 7.6 that I'm being somewhat paranoid about it.

I have several machines to upgrade, but so far the only one I've dared
to work on (least critical if it goes bad) is my new-to-me-but-used
laptop.

First I did a 'dd' backup of the whole drive onto an external USB drive
then did the upgrade. Since I use the Mate desktop, I enabled the
epel testing repo (per postings on Mate troubles at the time of the
upgrade).

Result: Boots to GDM just fine, but Mate is a black screen. Switching
to Gnome, works fine, but I can't stand Gnome.  Not knowing what else 
to try I restored the dd backup.

Later I inserted a spare HD and did a new install from the DVD
media. This installed without issue, but now no desktop works.
Switching to a console (CTRL-ALT-F2) I did a full "yum update",
which installed quite a few packages, but upon reboot nothing had
improved. So I added the epel repo (and testing) and installed Mate.
Ditto, nothing had changed.

This laptop is fairly old, and according to lshw, has a RV635/M86
[Mobility Radeon HD 3650]I] video chipset, in case it matters.
CPU is identified as: Intel(R) Core(TM)2 Duo CPU     T9400  @ 2.53GHz

Having previously installed 7.5 on it without trouble, on which Mate
works just fine, I wonder what could have become so broken that neither
an upgrade nor a fresh install works.

I have, as I said, several other machines to  upgrade, too, some of them
have Nvidia grahics (esp. my main desktop box, which also uses Linux
software RAID-1) and all run Mate. Then there is a netbook (low priority)
a couple of VMs (in virtualbox) and at work several other VMS as well as
bare-metal on a Lenovo deskside machine, and I'm afraid to do an update
on all of them because I need them to be in usable condition. The Lenovo
box in my office is somewhat urgent, as I am about to retire and turn
it over to someone else who doesn't know much about Linux.

I'd appreciate any advice I can get on how to get these systems upgraded
without breaking them.

thanks in advance for your help!

Fred
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