Hitler Dead
The only way I can see Germany surrendering before Berlin falls is if Hitler dies earlier. Perhaps have Operation Valkyrie actually work as far as killing Hitler. The Allies still demand unconditional surrender but Germany survives and may even get slighly--and I emphasize slightly--better terms.
Hitler, was the reason for the Battle of Berlin. He was determined that his end would be a Gotterdamnerung worthy of Wagner's operas. Berlin would be the place where this would happen. If Hitler is removed it is likely his generals, who by now knew the war was lost, could surrender Germany and hope for better treatment by both sides.
Geon