Before proceeding to the issue, raised in another thread, of the purpose of the 1991 modification of the order of succession to the throne, I think it will be useful to post the relevant parts of the Constitution of the Kingdom of Belgium.
From 1893 until 1991, Article 60 of the Constitution read as follows:
The constitutional powers of the King are hereditary through the direct, natural and legitimate descent from H.M. Leopold, George, Christian, Frederick of Saxe-Coburg, from male to male, by order of primogeniture and with the permanent exclusion of women and of their descendants.
The prince who marries without the King’s consent or, in his absence, without the consent of those exercising the King’s powers in cases provided for by the Constitution shall be deprived of his right to the crown.
Nonetheless, this right may be restored by the King or, in his absence, by those exercising the powers of the King in cases provided for by the Constitution, but only with the assent of both Houses.
In 1991, Article 60 was amended to read:
The constitutional powers of the King are hereditary through the direct, natural and legitimate descent from H.M. Leopold, George, Christian, Frederick of Saxe-Coburg, by order of primogeniture.
The descendant mentioned in the first paragraph who marries without the King’s consent or, in his absence, without the consent of those exercising the King’s powers in cases provided for by the Constitution shall be deprived of his right to the crown.
Nonetheless, this right may be restored by the King or, in his absence, by those exercising the powers of the King in cases provided for by the Constitution, but only with the assent of both Houses.
A note: The numbering of Article 60 has subsequently been changed to Article 85, but the wording of the article has remained identical ever since the reform of 1991.
As can be seen from the passages above, the modifications were (1) in the first paragraph, the deletion of the words "from male to male" and "the permanent exclusion of women and of their descendants", and (2) in the second paragraph, the substitution of the word "prince" with the words "descendant mentioned in the first paragraph".
Simultaneously, the following "transitional provision" was inserted into the Constitution (see Title IX):
I. – The provisions of Article 85 will for the first time be applicable to the progeny of H.R.H. Prince Albert, Felix, Humbert, Theodore, Christian, Eugene, Marie, Prince of Liege, Prince of Belgium, it being understood that the marriage of H.R.H. Princess Astrid, Josephine, Charlotte, Fabrizia, Elisabeth, Paola, Marie, Princess of Belgium to Lorenz, Archduke of Austria-Este, is regarded as having obtained the consent described in Article 85, second paragraph.
Until such time, the following provisions remain in effect.
The constitutional powers of the King are hereditary through the direct, natural and legitimate descent from H.M. Leopold, George, Christian, Frederick of Saxe-Coburg, from male to male, by order of primogeniture and with the permanent exclusion of women and of their descendants.
The prince who marries without the King’s consent or, in his absence, without the consent of those exercising the King’s powers in cases provided for by the Constitution shall be deprived of his right to the crown.
Nonetheless, this right may be restored by the King or, in his absence, by those exercising the powers of the King in cases provided for by the Constitution, but only with the assent of both Houses.
https://www.lachambre.be/kvvcr/pdf_sections/publications/constitution/GrondwetUK.pdf