In the Natural 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[Natural]):

:-convert('m', 'units', 'ft');  # the 'm' is not interpreted as a meter

Error, (in `convert/units`) unable to convert `1` to `ft`

convert('m', 'units', 'ft');

$\frac{1250}{381}⟦\mathrm{ft}⟧$