Store a version number with your save.

[System.Serializable]
public class SaveData
{
    public int version = 2;
    public int coins;
    public int level;
    public int experience;
}

Migrate old saves:

if (data.version == 1)
{
    data.experience = 0;
    data.version = 2;
}

Versioning helps preserve compatibility when your save structure changes.