Basically, when new releases occur can I just issue a slackpkg upgrade-all (after reading what the changes are and being aware of whats new etc) or is it recommended/required to download the latest major release and re-install the whole system?

This question is answered, as well as any related questions to the topic, in the slackpkg documentation on docs.slackware.com.

http://docs.slackware.com/slackware:slackpkg

http://docs.slackware.com/howtos:sla...using_slackpkg

"slpkg upgrade-all" is not all you need to do.

Yup. You can use slackpkg to go from one release to the next. Here is some reading for you.

http://docs.slackware.com/howtos:sla...:systemupgrade

Slackware doesn't hold your hand like Debian or Ubuntu would do. So the answer is YES, BUT... there's quite some stuff you have to do manually. Here's how.

https://github.com/kikinovak/slackwa...rade-HOWTO.txt

to do it correct way is to edit your /etc/slackpkg/mirrors to the next release.
slackpkg update
slackpkg install-new
slackpkg upgrade-all

remember to read the UPGRADE.TXT and CHANGES_AND_HINTS.TXT
and see what programs you want to manually remove.

remember if you have any of your packages blacklisted
in your /etc/slackpkg/blacklist then comment them out.
after the upgrade go through your slackbuilds and upgrade them also.
if using the SBOPKG browser to install make sure to set it to your new release.

if doing this with multi arch alien bobs multi-lib
you may want to read this
http://www.linuxquestions.org/questi...4/#post5224104