the point which you keep avoiding is that it should be possible to revert all payments if people do not want to accept your new conditions.
Which payments? Payments made through GLBSE? or payments made to upgrade from Gigamining to Terramining?
People that do not want to accept the new conditions also do not want to upgrade to Terramining.
Which payments? ALL payments (price of shares minus dividends paid) have to be paid back.
For example, suppose I sell my old car to my car dealer and buy a new car from him. Then the car dealer calls me the next day and informs me that he cannot sell me the new car because he just found out about a law prohibiting him to sell it to me. What would you do? Give him back the new car and go home? Of course not, you would return the new car (Gigamining contract) but you wouldn't accept the car dealer to keep the money your paid or keep your old car (dividends).