The provisions of section 23 of this Act (co-ownership of registered mark) apply as from the commencement of this Act to an existing registered mark of which two or more persons were immediately before commencement registered as joint proprietors.

But so long as the relations between the joint proprietors remain such as are described in section 63 of the 1938 Act (joint ownership) there shall be taken to be an agreement to exclude the operation of subsections (1) and (3) of section 23 of this Act (ownership in undivided shares and right of co-proprietor to make separate use of the mark).