在NearProtocol中，如何迁移合约状态

假设有一个用near-sdk-rs编写的合约，已部署，其状态定义为：

#[near_bindgen]
#[derive(BorshDeserialize, BorshSerialize)]
pub struct NFT {
    pub tokens: UnorderedMap<TokenId, Token>,
}

#[derive(BorshDeserialize, BorshSerialize)]
pub struct Token {
   pub owner: AccountId
}

现在这个合约有一些使用，因此有一些记录tokens存储在链上。 然后我想通过添加一个字段来更新此合​​同Token:

pub struct Token {
   pub owner: AccountId
   pub name: String // For existing ones, this will be set to ""
}

如何在保留现有状态的情况下执行此操作（类似于进行数据库迁移）？

您还可以看到我们创建的一些示例如何使用版本控制。

参见莓果俱乐部：

#[derive(BorshDeserialize, BorshSerialize)]
pub struct AccountVersionAvocado {
    pub account_id: AccountId,
    pub account_index: AccountIndex,
    pub balance: u128,
    pub num_pixels: u32,
    pub claim_timestamp: u64,
}

impl From<AccountVersionAvocado> for Account {
    fn from(account: AccountVersionAvocado) -> Self {
        Self {
            account_id: account.account_id,
            account_index: account.account_index,
            balances: vec![account.balance, 0],
            num_pixels: account.num_pixels,
            claim_timestamp: account.claim_timestamp,
            farming_preference: Berry::Avocado,
        }
    }
}

https://github.com/evgenykuzyakov/berryclub/blob/ad2b37045b14fa72181ab92831fb741a7c40234b/contract-rs/pixel-board/src/account.rs#L19-L39 https://github.com/evgenykuzyakov/berryclub/blob/ad2b37045b14fa72181ab92831fb741a7c40234b/contract-rs/pixel-board/src/account.rs#L19-L39

还有其他的，但一旦我找到它们，我就必须回到这个