Yes Sir! Varying a scheduled Day Off is clearly illegal under the Hong Kong Employment Ordinance (CAP 57).
To whit;
CAP 57
18. Appointment of rest days
(1) Rest days shall be appointed by an employer and he may
appoint different rest days for different employees. (Amended
71 of 1976 s. 4)
(2) Subject to subsection (4), every employer shall, before the
commencement of every month, inform each employee orally
or in writing of his rest days in that month.
And:
19. Compulsory work on rest days
(1) Subject to subsection (2), no employer shall require an
employee to work on any of his rest days.
(2) An employer may require an employee to work on his rest
day if it is necessary to do so by reason of a breakdown of
machinery or plant or other unforeseen emergency of any
nature.
The CAD may not be interested in enforcing the HKEO, but they might show some more regard for it.