They are titled "Important changes may affect how you earn rewards".

I've only received a few of them and there's no obvious pattern, except that they are very much expired. (Not by date, by not being valid at all.) Certainly doesn't seem to be anything to do with the first digits of a card number. For example, I had an Amex which I changed to a different type. It was cancelled and a new one issued. The only thing which changed was the last 4 digits. The old version was one of the cards RN emailed me about. The new version was accepted by RN.