Peace agreements concluded after internal armed conflicts often stipulate radical constitutional change. When this is envisioned to take place within an existing constitution, it creates normativity clashes between the constitution and the peace agreement by circumventing constitutional amendment procedures or conflicting with the unamendable constitutional principles. This paper sets out the manifestations of such clashes of normativity by a comparative study before focusing on the ongoing peace processes in the Philippines and Colombia. By reference to the case studies, the paper conceptualises potential modalities of coping with such unconstitutionality challenges and examines these modalities in four groups: (i) linking the legality of a peace agreement to international law (ii) transitional constitutionalist interpretation (iii) transitional constitutional instruments or deferral (iv) enhancing process legitimacy to increase the legitimacy of the proposed constitutional change.