Hiring Date Issue

hi sap guru's,

i have a problem..actually i have a peronell number whoose joinig date is 22.03.1994 but in my sap system hiring date is 22.03.2002 which is in correct so now i have to change it but system is not allowing..can anyone please tell me that how would i change it? would there any effect on other data if i change it? actually this person is going to retire in next year. so please help me....

regards saurabh.