I have an update on this topic for 1945. There is a very good review of changes to length of tour in 1945 in W.S. Carter, "Anglo-Canadian Wartime Relations, 1939-1945: RAF Bomber Command and No. 6 (Canadian) Group" (PhD) McMaster University, available for download here:
The relevant section on increase of tour length to 36 sorties, February 1945 begins on page 214 (of 249 pages) (PDF page numbering). Dad wrote home in February 1945, telling my Aunt of the change. Dad was screened after 30 operations, end of March 1945. The review is interesting because it had to do with the closing of various OTU and HCU stations, and the transfer of airmen to the navy (17,000) army (20,000) and transport command (260 crews). The decision by Harris to increase the length of the tour was not made in consultation with the Air Ministry and the Secretary of State for Air, prior to the receipt of Ministerial Approval. The RCAF was "frosted" about this because it was a direct violation of the Balfour-Power Agreement which stipulated that policy changes in conditions of service of aircrew a matter of intergovernmental consultation.