Depends on the wheel back there. Did they swap in a different hub with a shorter axle? Or is it a 130 or 135mm long axle which was normal place for MTBs? That will determine what you can do. Measure the space between the locknuts. If it's greater than or equal to 126mm, you're good to go.
Swap your choice of 6 or 7 spd freewheel, adjust the limit screws of your derailleur and verify you have enough chain length for the low gear of the new freewheel.