No clue were you have read this, but it was no suggestion from an openSUSE documentation.
You should learn (and try to understand) the differences between the following two commands:
Correct example for use of --allow-vendor-change:
sudo zypper dup --from packman --allow-vendor-change
Generally wrong and dangerous application of --allow-vendor-change:
sudo zypper dup --allow-vendor-change
The first command applies the vendor change to only ONE particular repository. GOOD!
The second command applies the vendor change to ALL of your repositories which is potentially BAD! Dependend of how many repos (home, OBS, 3rd party, beta, …) you use, you will break your system in seconds…
And this explanation (in terminal form) was already given in the first answer in this thread by Sauerland 