I've had this too, and have also reported it. I'm guessing it's because not just the IBAN has changed, but the Bank Identifier Codes and Branch BIC codes have as well, but the sort code, accord number and physical branches have not changed. At some point they'll be looking up this connection and it just doesn't make sense according to what their database offers. Things like the BICs have changed, but nowhere on iTunes Connect can you enter this, so I suspect they're looking this up, and this no longer matches the new IBANs. Whether this is HSBCs fault by doing something non-standard, or Apple trying to make things easier and not dealing with this scenario is for now unknown. I've even tried creating/adding an additional bank account record in iTunes Connect and this also failed in exactly the same way.
IF Apple pay us from a UK bank account, which just uses the Sort Code and Account number, maybe this won't matter, but if they pay us using an international account it could be a disaster for those of us using the 'wrong' bank.