The berlin corridors were established during the blockade and were at heights and speeds commensurate with Yorks, Dakotas and Lancastrians of the time. These became set in concrete with a tacit agreement not to rock the boat.
The corridors were what the allies established and the Russians didn't prevent. I don't know if the agreement was then formalised.
I think you will find they pre-date the blockade. My understanding is they were established in 1945 under 4-power agreement. Certainly they had a top altitude of around FL100 right up to the time of their demise at the end of the Cold War. Any change would have needed the agreement of the same 4 powers, and of course the FO would not ask because the inevitable refusal would have offended them. During the 80s I flew the centre corridor frequently on trooping flights to Gatow - very interesting - especially for my copilot one day, who got a clear view of a place that had been of earlier operational interest to him!