A new plan to rescue Union Station and possibly turn it into a commercial center was approved without opposition yesterday with only technical changes by a Senate subcommittee. Supported in its basics by the Reagan administration, the plan now goes to the full Senate.

Staff sources foresaw no serious opposition when the plan's bill comes to a vote, possibly within two weeks. It then would go to the House, where the outcome is less clear.

The plan would finish on-going roof repairs and a parking garage and study what further renovation the station needs and the feasibility of developing it commercially. It would move control of the station to the U.S. Department of Transportation from Interior.

The bill would empower the Transportation Department to buy the station from railroad interests that own it and, if feasible, sign with private developers, who might build shops, restaurants, and other facilities. Trains would continue to operate at the station.

The Senate plan is the latest congressional bid to rescue the deteriorating station, now known officially as the National Visitor Center. Concern soared anew in February after flooding and falling plaster led officials to declare it a safety hazard and close it down.

Except for up to $1.7 million authorized for the building purchase, the plan would use government funds already appropriated for other programs and private money.

The parking garage would be finished with $40 million in federal funds appropriated for interstate highways that the District of Columbia plans to cancel. According to D.C. Transportation Department chief Tom Downs, the city must spend the money in 1982 for a qualifying project, such as the garage, or lose it.