Without making any other changes:
- Snap the Now time to 51:01:000 (current tempo + 1 because measures aren't numbered from 0).
- Hit Shift+M to open the Set Measure/Beat At Now dialog.
- Enter measure 81, beat 1, and OK.
- SONAR will automatically make the tempo change, shifting the timeline around existing MIDI and audio clips without altering their absolute playback timing or tempo.