According to Rigify's Doc:
Nondisruptive re-rig If the generated rig doesn’t fit all the features you need or, for example, you decide to add something more to your character (like a sixth arm or a tail), you can re-generate your rig without losing your previously generated features and your animation data.
But what does this exactly mean? I tried this simple finger pose:
And I regenerate rig from my metarig. All the animation data is gone:
How can I re-rig without losing my animation data?