Change / Extend Background in Video

Greetings! I have a stable video of musicians performing against a background picture. The camera does not move, only the subjects. The band are inside the background most of the time but sometimes limbs, arms or hair move out. I need to extend the background behind the band to fill the screen, but make it as close as possible to the original look of the video. Certain things would have to be masked / rotoscoped frame by frame. There are 3 different videos, only about 2.5 minutes each of the same performance from different angles. I would need all 3 videos worked on and output at high resolution with no quality loss. Please see the attached image. I can provide a high resolution still image of the background to be used. Thank you!