On May 20, USCIS will begin premium processing for FY 2020 cap-subject H-1B petitioners requesting a change of status on their Form I-129, Petition for a Nonimmigrant Worker. So, if you have already received an electronic receipt for your H-1B petition, the 15-day clock will not begin until 5/20/2019.
Employers who did not file Form I-907, Request for Premium Processing Service, concurrently with an FY 2020 cap-subject H-1B petition requesting a change of status must wait until premium processing begins on May 20 to submit Form I-907.