I had the same doubt before. Later I found there is technical reasons after I compared the configuration files.

In fact the root cause is that the new firmware adopts a new HTTP authentication method. The old firmware including Archer C7_V1_140704 uses HTTP Basic Auth, which pops up a small windows to ask users to enter username and password for login. The username/password is save and sent in plain text .

The firmware Archer C7_V1_141204 starts using a more secure method to implement HTTP authentication (we can see this by capturing). The username/password is encrypted which means the format is changed. If we restore the old configuration file after firmware upgrade, it can be restored but the password is unrecognized.

Download the tool which can view the configuration file in plain text. http://www.nirsoft.net/toolsdownload/routerpassview.zip

But I can't find a way to modified. Not only this brand, a lot of other brands encrypt their configuration files for safety.

I had a problem logging back into the admin after the firmware upgrade as well. I called TP-Link Tech support (866) 225-8139. The fix was simple log into the admin by using the default setting of User ID admin and password admin. That will let you into your account and from there you can do what is needed like changing your ID and password. You can a reboot there as well. Hope this helps.

It's a good thing, if you had some Mal that the new FW wiped out you wouldn 't want a re-infection through an old config file would you.