On the face of it, it is a significant decision but I do wonder how much practical difference it will make. The Hagia Sophia is basically a mosque anyway, only with world class calligraphy and art exhibitions. It is not clear yet whether these will be slung out.
The nearby Blue Mosque is a proper mosquey mosque, but they really don't do great business, apart from amongst tourists. In fact, they are both on the peninsular which, to my eyes seems, pretty secular. Further up the Golden Horn, to the north west, there's Ayakapı which is a solid, working class, deeply religious area with a mosque on every corner and where you can't get a Raki for love nor Lira. I can't see those people traipsing over to the Hagia Sophia on a regular basis, especially when their current mosques are community hubs where everyone knows each other.
But, yes. This move is symbolic and we ignore politically symbolic moves at our peril.