It looks here are 2 things mixed up
CUNI is about unit of measures,
the links and screen shots given are related to currency settings.
in CUNI you can only define how the decimals are presented to the user, but stored in tables is always with 3 decimals.
from OSS note 23771:
.......inventory management is always carried out in base unit of measure and quantities are managed with exactly 3 places behind the decimal point........
Hence a change does technically not really change much, it may uncover decimals which were unknown because you never saw them before, but SAP always had them in the table if the user posted fractions