page 3 of the memo:

"For example, a hu*****and and wife who come to the United States as a principal H-1B and dependent H-4 spouse may maintain status for six years, and then change status to H-4 and H-1B respectively. Note that, upon the switch, the new 'principal alien' would be subject to the H-1B cap if not independently exempt. USCIS will consider, in the context of any applications for change of status from H-4 to H-1B, whether the H-4 alien complied with the requirements of accompanying or joining the H-1B alien, and whether the alien otherwise maintained valid nonimmigrant status."