On the date a district, as defined in subdivision (l) of Section 31468, is included in the retirement system, any personnel appointed pursuant to Sections 31522.5, 31522.9, 31522.10, and 31529.9 who had previously been in county service shall continue to be members of the system without interruption in service or loss of credit. Thereafter, each person entering employment with the district shall become a member of the system on the first day of the calendar month following his or her entrance into service.