aarch64 is an ARM 64 bit system (kernel), while arm7l is 32 bit ARM.
Can you convert from System: rpi4 (aarch64) to System: rpi4 (armv71)?
It depends. If you installed your system from the beta 64bit Raspberry Pi OS image, then no, you cannot convert. If you merely modified the /boot/config.txt file on a 32 bit Raspberry Pi OS and added the option to boot the 64bit kernel (arm_64bit=1), then you can remove that configuration line and you'll be booted back to a 32bit kernel/OS (arm7l).
