actually make old save compatible with new update is not that easy in some ways.
its easy if you did your script well and not forget anything. or if no specific values/script have ben modified in the new udpate. but...
i take exempl, if you forget to add , or didnt planned at base, to make a choices to have effect in futur update. your stuck. fixing this is only possibe by breaking old save.
reason is, you can't add a script or allowing to change variable. because you cant decide for the player.
wich choice the player have made before ? you cant know that before doing the update, soo you'r stuck and variable is stuck too. unless, like i say, you force the player to a choice that is not necessary his own choice. all you can do is to give the player another try to make the exact same choice or asking him wich choice he made before.
or you break the old save.
downward compatibility is not something that you can do by moving a finger. its more complex.
but yeah sure... if its that easy, then you welcome to make a patch by yourself and fixing stuff. without asking players wich choices they made before and something that is compatible with every players choices. good luck with that. XD