Here is a way to flash Sonoff-Tasmota to several Sonoff devices without need to opåen or solder the device. It uses Sonoff´s own OTA update system.
https://github.com/mirko/SonOTA
I flashed my Sonoff RF wi-fi/433mhs device with this, and it did work allmost perfectly. It didnt try to connect to given network after flashing.
What needed to do more was to set wifi hotspot to phone, for sonoff to connect.
SSID of the hotspot needs to be "indebuurt1", and password "VnsqrtnrsddbrN", Sonoff RF connects to it and you need to find its ip. I did use network analyser app from play store. Browse to its ip, log in to change wifi information to your needs
Working Sonoff devices : https://github.com/mirko/SonOTA/wiki
-Tontze