Comment by FabHK

Hehe, yes, but the point is this:

if you had verified fingerprints with Bob and are happily chatting with him, all the messages that reached him (two tick marks in WhatsApp) are safe.

Only those that have not yet been delivered (one tick mark) would, when the server sends you you a new key, be re-encrypted and re-sent.

All of this, as usual, is predicated on the client behaving as promised.