Tens of thousands of lower income riders who rely on greater Boston’s public transit system could soon be getting a break on their fares under a plan approved by the Massachusetts Bay Transportation Authority’s board of directors. The proposal adopted Thursday would offer riders who are aged 26-64, non-disabled, and have lower incomes reduced fares of about 50% off all MBTA modes, including subway, bus and commuter rail. The fare changes are expected to go into effect this summer. The MBTA estimates the cost of the changes to be up to $62 million including administrative costs and fare revenue loss.