New York City officials announced a plan on Wednesday to change bail requirements for some low-level offenders in an effort to keep thousands of people accused of nonviolent crimes and misdemeanors out of the troubled Rikers Island jail complex. The program, which is expected to cost nearly $18 million, will allow judges to release up to 3,000 low-risk defendants while placing them under court supervision as they await trial. Supporters of the program hope the initiative will help defendants who otherwise would remain jailed because they cannot afford bail.