Just upgraded Linksys MR8300 to 19.07.4 using EA8300 factory.bin. I used Advanced Reboot https://github.com/openwrt/luci/blob/master/applications/luci-app-advanced-reboot/README.md, thanks to Flashing OpenWrt on Linksys MR8300 - partially bricked, luci module to boot back into OEM partition and reinstalled factory.bin with http://routeripaddress/fwupdate.html web firmware uploader and reconfigured once rebooted.
All is working fine !
Many thanks to Dev Team !
Edit: Also upgraded Linksys EA8300, booted from OEM partition, upgraded with factory.bin, reconfigured, all is fine.