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).
How much did you pay Gigavps for your bonds, for how many bonds, and how much in dividends have you been returned? If you can confirm that you bought shares directly from Giga then comparing Giga to a car dealer and Gigamining to a (leased) car is reasonable. If you did not buy bonds directly from Giga, then you're out of luck as there is no Kelly Blue Book of mining bonds.
I'm having a hard time understanding the example you are proposing and the point you're trying to convey. A lease situation would make more sense.
I bought my shares directly from giga. If he doesn't want to/can't honor the original contract he could easily revert it. Of course I got nothing.