Is a Qualified Digital Certificate backward compatible with existing certificate code? I'm trying to determine if our digital signature generation code will work without change using a European Qualified Digital Certificate in place of the RSA signing certificate that we normally use.

It seems like the structure of the qualified certificate is the same as any other signing certificate, but has special rules around how it is generated. If this is the case, then I suppose existing certificate handling code should continue to work. But I haven't found actual confirmation of this theory, and I don't have a QDC in hand to try it.

The code in question is a Windows-based C++ application using the Microsoft Cryptography API functions. I also asked the question with code example on stackoverflow.com.

Anybody have experience with this?
TLDR: yes.

Qualified Digital Certificate is not a specification aiming at insuring interoperability. It purposely does not even specify a precise cryptosystem (see comment). Thus in principle there is no sure answer to that question.

However Qualified Digital Certificate only requires common X.509 features, modern Microsoft Cryptography API is flexible in what it accepts, and because it is common certificate issuers typically take care of going by whatever requirement it sets. Thus my bets are on: likely yes, and I'd be surprised if a little care in parametrization at key generation and procurement of the certificate did not make it possible.

Note: Qualified Digital Certificates does not imply Qualified Digital Signature, and there is a long way from the former to the later.