In the Standard Units environment, the global convert function is replaced by a convert function that converts any unevaluated arithmetic operators, equalities, or inequalities to their global equivalents.  The first argument is tested to check whether it is a valid unit name.

For more information about this function, see the global convert help page.

with(Units[Standard]):

:-convert(3*Unit('m/s'), 'units', 'cm/s');  # this should fail

Error, (in `convert/units`) `Units:-Standard:-`*`(cm,Units:-Standard:-`/`(s))` is not a recognized unit

convert(3*Unit('m/s'), 'units', 'cm/s');