WASHINGTON -- One of the most wide-reaching and significant changes to military pay and benefits over the last 70 years goes into effect Jan. 1 with the implementation of the Uniformed Services Blended Retirement System, known as BRS.The new system blends aspects of the traditional defined benefit retirement pension system, with a defined contribution system of automatic and matching government contributions through the Thrift Savings Plan.All new entrants into the uniformed services on or after Jan. 1 will be enrolled in this new retirement system, Pentagon officials said. The uniformed services are the Army, Navy, Air Force, Marine Corps, Coast Guard, Public Health Service Commissioned Corps and the National Oceanic and Atmospheric Administration Commissioned Officer Corps.SOME CAN CHOSE BETWEEN SYSTEMSNearly 1.6 million current service members will have the option to remain in the current legacy "high-3" retirement system or to choose the BRS when the opt-in period for eligible service members opens Jan. 1. Opt-in eligible service members from all seven of the uniformed services will have an entire year to make their retirement system election. The open period for the majority of service members is from Jan. 1 through Dec. 31, 2018.Service members will need to visit one of these designated resources to opt into BRS:-- Army, Air Force, Navy: MyPay (https://mypay.dfas.mil).-- Marine Corps: Marine Online (https://www.mol.usmc.mil).-- Coast Guard, NOAA Commissioned Corps: Direct Access (https://portal.direct-access.us).-- U.S. Public Health Service personnel should contact the USPHS Compensation Branch.Service members who believe they are eligible to opt in, but do not see the opt-in option available online should contact their local personnel/human resources office to verify eligibility, officials said.DECISION IRREVOCABLEThe decision to opt in is irrevocable, officials emphasized, even if a service member changes his or her mind before the Dec. 31, 2018, deadline. Eligible service members who take no action will remain in the legacy retirement system, they added.Prior to opting in, officials recommend that service members take advantage of all available resources to assist in making an informed decision on the financial implications specific to their retirement situation. The Defense Department endorses several training and informational tools to support a service member's decision, including the BRS Opt-In Course, the BRS Comparison Calculator and numerous online BRS resource materials. Service members can receive no-cost, personal support from an accredited personal financial manager or counselor available at their installation's military and family support center or by calling Military OneSource at 1-800-342-9647.