FILIPINA domestic helpers have been warned against changing employers illegally - even if the change is brokered by an established job agency. More and more foreign domestics arriving in Hong Kong on a contract basis were either quitting their jobs or moonlighting through registered employment agencies, said Remy Borlongan, the head of the Asian Domestic Workers Union. Most of these helpers arenow illegal immigrants. ''I do not know exactly how many foreign domestic helpers are making money out of jobs not stated in their contract, because moonlighting of foreign domestic workers is illegal here,'' said Ms Borlongan. ''Although we do not have an official record, I think there are close to 1,000 of them doing this.'' The chairman of the United Filipinos in Hong Kong group, Marrz Balaoro, said most of the more than 1,000 local licensed agencies give alternative jobs to foreign domestic helpers who have visa problems when their contracts are terminated. ''Almost all employment agencies provide these jobs,'' said Ms Balaoro. ''The employment agencies see when the domestic workers are in a state of helplessness. They then recommend a sponsorship scheme to the workers and charge them between $7,000 and $12,000.'' More than 220 domestic helpers went missing before the end of August this year, compared with the same number for the whole of 1992. These maids are believed to be making a living by working as chars on an hourly basis for several employers. Once employment contracts are nullified, these workers are required by law to go back to their home countries within two weeks, but many choose to stay even though it is illegal. According to the Labour Department, many of Hong Kong's 114,000 foreign domestic helpers have problems with their employers. During the first nine months of this year, the department received nearly 1,500 cases of disputes between domestic helpers and their employers.