Beginning this weekend, subway trains running after 9 p.m. Sundays through Thursdays on the MBTA’s Orange Line between Oak Grove Station in Malden and Sullivan Station in Charlestown will be replaced with bus service, the transit authority announced. The shift to bus service, which will continue through Dec. 28, will allow workers to continue progress on the $29.2 million station at Assembly Square, which will provide transportation connections for the planned Assembly Row development also under construction. The development will include 2,100 apartments and 600,000 square feet of retail space. The new station is expected to be completed by 2014, according to the MBTA.