After reading some background material, it seems Saied's argument(s) for doing this is that the existing constitution, which was semi-presidential in nature, was not really working.
Besides the political deadlock which is actually somewhat common is such systems, Tunisia had more serious problems in that regard, in that a Constitutional Court, which was supposed to decide on the [ultimate] legality of measures, including decrees, could not even be appointed due to the deadlock.
Now old constitutions that were found rather inadequate were sometimes cast aside with little consideration for their provisions, including the first one of the US. But in the US case that was done by an assembly very much like the one that drafted the initial one.
On the other hand, would-be autocrats consolidating their power by drafting a new constitution and putting it to a referendum in less than fair circumstance, e.g. control of the media, is a much more common development, including what happened in Belarus after the fall of the USSR. Given the police raid on Al Jazeera that let to the closure of its office in Tunisia, that looks more likely to be the path the Saied is taking, at the moment.
As far as I can tell, he hasn't announced in any detail what exact constitutional reforms he envisages, or how he plans to carry them out (e.g. if he even intends to hold a referendum) which is perhaps one reason that most countries haven't had much of an official reaction besides expressing concern at the developments in Tunisia.
Only Turkey, which fears an increased influence of the Saudi/UAE model in Tunisia, seems to have taken a stronger stance insofar.... although that was seemingly limited to some actors in Turkey:
Turkish President Recip Tayyip Erdogan and his ruling Justice and Development Party (AKP) share close ideological and friendly ties with Ennahda's leader Rached Ghannouchi, the [Tunisian] parliament speaker, who hasn't been able to enter his official office since the takeover.
Even though AKP spokesperson Omer Celik called the power grab a coup, both Erdogan and Foreign Minister Mevlut Cavusoglu have steered clear of the term.
Turkey itself has had some authoritarian developments in the past decade... and the irony here is that Turkey itself has transitioned towards "ultra-presidentialism". This was also done by a referendum.