Just be more clear, for AR750S, upgrade from old firmware (3.104) which is based on openwrt 1806 to latest firmware 3.201 which is based on openwrt 1907 could cause verification failed.
In 3.105 (openwrt 1806) we made special treatment so that it can be upgraded to openwrt 1907 and later.
So you may upgrade to 3.105 first then upgrade to 3.201.
Or you can use uboot to upgrade to 3.201 directly without keep your settings