If you readjust the rear wheel forword why wouldn't you just go ahead and take the chain off the rear sprocket? I think the OP's question was without removing the rear tire or changing the slack adjusters. So the answer I would give is no. If the chain is properly adjusted there should not be enough slack to change the front sprocket. IMHOIf you loosen the rear wheel and move the adjusters forward some there is plenty of room to remove the front sprocket.