SURELY ALMOST everyone has imagined, if only for a moment, what it would be like to become someone else, to change one's identity. People accepted in the Justice Department's Witness Security Program have a chance to live this fantasy. The government now tells some witnesses who are reluctant to testify out of fear for their lives that it will protect them by helping them acquire new identities and new jobs in communities where they are not known. The Justice Department spends some $28 million on witness protection and processes some 300 new witnesses a year. Since 1970, more than 4,000 witnesses and 8,000 family members have been given new identities.

But there are problems. Many of the people available to testify in organized crime cases aren't such nice folks themselves; seven such witnesses have been convicted of murder after their relocations, and many have committed other crimes--the Justice Department hasn't bothered to keep track. And many relocated witnesses or their spouses avoid paying debts and keep children away from other parents who have rights of visitation or even custody. Consider the case of William Franz, divorced in 1977, whose former wife took her children with her after she married a witness who got a new identity. Mr. Franz has not seen his children since 1977; the government allowed him to send them letters, but he has had no replies. He brought suit, and a federal appeals court ruled last week that the freedom of a parent and child to "maintain, cultivate, and mold their ongoing relationship" is protected by the Constitution.

Both the Justice Department and Congress should have done something about such cases long ago. In response to a Government Accounting Office investigation, the Justice Department in 1982 did agree to serve legal papers on relocated witnesses sued by creditors or in child custody or visitation cases. But if the witness refuses to comply with a court judgment, the Justice Department hasn't done anything. Until 1982, it refused to disclose the new identities of witnesses under any circumstances. Then it accepted a GAO recommendation that disclosure be made sometimes, but only when the Justice Department agreed. It opposes a GAO recommendation that courts should be able to overturn decisions not to disclose new names.

Presumably the Justice Department fears it will be harder to get witnesses to testify if it doesn't have full power to insulate them in their new identities. But why should witnesses be able to evade lawful court orders--and break up families? The GAO has it right. Congress should pass a law authorizing disclosure of witnesses' names in certain circumstances and making such decisions reviewable in court. Witnesses should be told they won't be protected if they don't pay their debts and obey child custody and visitation orders. In fantasy it may be all right for a person with a new identity to evade obligations, but not in real life.