[pfg]

You should add one additional (CA|end-entity) pin regardless. If your key is compromised, there's a non-zero chance that you'll realize that your CA has stopped issuing certificates under the CA key you pinned at the same time. Most CAs aren't very transparent with regards to their root/intermediate lifecycles.

[hdhzy]

Well actually HPKP will not work if there is only one pin present. The browser will reject the entire header.

[pfg]

I had to double-check the RFC on this. The OP suggested pinning to "current CA + current pubkey", which would technically count as two pins and satisfy a simple "you'll need at least two pins" requirement. Luckily, the RFC authors realized that someone would try this and wrote it as "The given set of Pins contains at least one Pin that does NOT refer to an SPKI in the certificate chain.", meaning this pin would get rejected.

[hdhzy]

Yes, although they cannot make people backup the inactive pinned key they at least get them to think what they are doing.