How to implement cards migration?

[PackRuble]

There is a distant idea that card migration could be implemented.
Why?

• was MyCard.index. We wanted to make MyCard.currentIndex. In the SP this lives with the index key, and the new desired key is currentIndex (let's say the naming matches the enumeration - it doesn't matter now). I'd like to add a deprecated annotation to the old card and create a new card + specify "something" for migration.
• previously we used the shared_preferences package. Now we would like to use Cardoteka and move all the old keys to the new addresses (re-saving the values of course)

I think it should be in CardotekaConfig as a field and passed as a parameter. Implementation can be done as a getter when defining MyCard's (or wherever). I would like to use a separate class, but in Map form I see it like this (pseudo code):

enum MyCard implements Card {
  ...values

  static get migrator => {
    'old_key': MyCard.newKey,
  };
}

There's an important point here: we want to transfer values to another key in SP (and delete the old key).

[PackRuble]

Also, add a section to the readme about card migrations (and key migrations with shared_preferenses)