I just did this.
My device had a password on the web page, and I did a firmware update on the 'wrong' device.
Ended up cutting power to my desktop, and loosing the password I just entered on the device in the process.
Yes I should have written it down somewhere safe (pw manager was not saved). Yes I should have verified that I entered the correct IP address, and not the one My PC was running off.
Result: I'm not trying to reset the password on my brand new 2 devices. :-/