::::::::::::::::::::A few cats for the pigeons to contend with. It is true that Jerusalem was not allocated to either state in the UNGA Res 181.  However, UNGA Res 181 only came into effect if a party or parties declared.   At the time the Mandate expired, Jerusalem was a part of Palestine.  Israel did not declare Sovereignty over any part of Jerusalem.  Confirmed in statements by the Israeli Government to the UNSC 22nd May 1948. Corpus separatum was not implemented. 

 ::::::::::::::::::::So, when did the status of Jerusalem legally change from being a part of Palestine?  It didn't. The Jewish areas were in Palestine. 

 ::::::::::::::::::::We can see this reflected in the fact that there are numerous UNSC Resolutions passed against Israel for attempting to "change the status" of Jerusalem. There is no UNSC Resolution condemning TransJordan for A) Taking control of Palestinian territories B) changing the status of territories it annexed as a temporary trustee. There's no UNSC Resolution simply because Jerusalem was still a part of Palestine. Under the UN Charter, Regional Powers have a right to take control of non-self-governing territories they represent and attempt to expel foreign forces.  Anyone who took up arms, even for self defense, Jewish or non-Jewish, became a belligerent