NOT in Tumbleweed. But for Leap, yes.
zypper dup syncs a user’s Tumbleweed installation with the contents of the latest available Tumbleweed rolling release, and isn’t guaranteed not to break things
Actually, zypper dup performs a distribution upgrade. Each Tumbleweed release is a new distribution. Therefore, you only use zypper dup, you do not use zypper up in TW.
zypper dup in Leap also performs a distribution “upgrade”, but unless you change the repos to a higher version, will instead actually “upgrade” your installed system from the same release (ie: 42.2 to 42.2, or 42.3 to 42.3) to the same release, but will switch your changed packages back to the openSUSE repos. Used that way, it can sometimes break things or even terribly break things, according to your install.
Thus, unless you are trying to repair a seriously broken system, or want to upgrade from 42.2 to 42.3, you generally do not use zypper dup in Leap.
zypper dup now performs the same function as zypper dup --no-allow-vendor-change
Only in Tumbleweed. It has been set, in Tumbleweed only, to use the --no-allow-vendor-change switch by default. In Leap, you will still have to specify that parameter when needed.
Tumbleweed won’t tell me to update or upgrade, at least not with the Cinnamon desktop. I must run zypper dup or zypper up.
It is not Tumbleweed that won’t tell you, it is the Software Updater that will not tell you. This is by design, since the updater does not run the mandatory zypper dup.
And, again, in Tumbleweed, use ONLY zypper dup, NOT zypper up.