Shaikh Salih al-Fawzaan said:

I say: the kufr of the one who does not rule by what Allaah has revealed, is not restricted to juhood, rather it also applies to complete replacement (bal, yatanaawal al-istibdaala at-taamm), and likewise, the one who makes istihlaal of this action, even if only in some laws, and without him making juhood, or if he says: the judgement of other than Allaah is better than the judgement of Allaah, or if he says: that they are both equal, as the people of knowledge have textually stated (concerning this). Until even if he said, that the judgment of Allaah is better, but it is permissible to judge by other than it...

Shaikh Salih al-Fawzaan also said:

This Tabdeel that you have mentioned and which is kufr by Ijmaa' of the Muslims, does not exist, but it is something that is assumed by you, none of the rulers of today speak of it and nor in the past, but there is Istibdaal, and this is choosing the Secular Laws as a replacement to the Islamic Sharee'ah, and abolishing the Sharee'ah law courts, and this is kufr also. This is because, the ruler completely removes (yunahheehaa nihaa'iyyan) the Sharee'ah, and puts in its place Secular Laws. So what then remains of Islaam? And he did not do this except that he adopts these laws while he considers them to be superior to the Sharee'ah.

Source: Majallat ud-Da'wah no. 1749 4th Rabee ul-Awwal 1421H.

And see these articles for further information on this matter: