I just upgraded an end-user laptop workstation from Ubuntu 18.04 LTS to 
Ubuntu 20.04 LTS.  Compared to what is involved in making a major 
release change in SL, this was trivial.

Because SL (and presumably all EL, perhaps except for some licensing 
from RH for which RHEL will "upgrade in place" -- at least I have read 
about such) requires a fresh install with a fresh format of the hard 
drive to change major releases (e.g., SL 6 to SL 7), I typically 
purchased a new harddrive upon which to install a new major release of 
SL.  After the partitioning and installation of the default SL N+1, I 
would place the SL N system harddrive into an external drive interface 
adapter, often via a USB connection to the running SL N+1 system, and 
proceed to copy files onto the new harddrive (not just /home, but /opt, 
/usr/local, etc.).  Once copied, some (many?) applications that existed 
in /opt, etc., (not /etc -- English usage, not a directory), would then 
have to be upgraded or older libraries had to be installed but not in 
any way that could interfere with the SL N+1 libraries needed by the SL 
N+1 system.  This could be a complicated and tedious procedure, often 
involving more than one day of work.

By contrast, going from Ubuntu 18.04 LTS to 20.04 LTS was an upgrade in 
place, over the web (but taking some time because of the limited 
bandwidth we have at home using our ISP).  The only precaution I did was 
to copy the end-user's complete home directory to an external USB 
harddrive.  I then proceeded with the upgrade using one of several web 
sources for the instructions.  The upgraded system boots, and other than 
a peculiarity in getting to one search engine URL from Mozilla Firefox 
non-distro (stock production from Mozilla), everything seems to work as 
well or better than the pre-upgraded Ubuntu. I have not needed to access 
any of the files on the external USB pre-upgrade backup.  During the 
upgrade, the upgrade utility detected that I had edited several system 
utility configuration files (that allowed such actions a root MATE GUI 
login and use screen, not just sudo within an ordinary user MATE 
screen), and these evidently were the "same" between major releases (the 
changes all seem to have been retained and are operable).  The upgraded 
included the use of the proprietary Nvidia GPU video driver and the 
Nvidia GPU proprietary settings/control GUI application (under MATE) 
that might, or might not, require an upgrade from a non-SL-hosted distro 
for the same type of major release upgrade on SL (having done this with 
that experience during several SL major release upgrades).

I am not using Ubuntu on any server at this time, although I know of 
several sites that do use Ubuntu LTS as a server OS.  However, I am 
using it on workstations because it seems stable and it seems to have an 
easier "time" of enabling "current" GUI production applications that are 
need for composing and viewing files of various types (e.g., TeXstudio) 
than does SL, mostly because of SL using an older c/c++ library that 
also is used by SL ("kernel") and thus not easy (nor safe) to change.

Take care.  Stay safe.

Yasha Karant