There wasn't exactly a widespread transition from straight bolts to curved. Some nations did not use straight bolts during WWI, like UK with their Enfield No.1 Mk. III and the US with their Springfield 1903s. Both of these nations continued using curved bolts during WWII. Other participants in WWII were content with continued use of straight bolt rifles, such as the Soviet Union with their Mosin-Nagant M1891/30 rifles and Japan with their Type 38 and Type 99 rifles. So while Germany's transition from the straight bolt Gewehr 1898 to the curved bolt Karabiner 98k is probably the most notable example of a transition from a straight to curved bolt, there doesn't seem to be enough of a trend to suggest that there was a general transition.