WAC510 admin password appears to have changed itself? One SSID still working, the other is not

We've been using four WAC510 APs in our office for a couple of years now with no issues. Obviously the admin password to login to the AP's configuration was set by me a long while ago. I hadn't logged into them for a long time but needed to today as our guestwifi is not connecting to the internet, and have found that my password is no longer working. The default 'out of the box' password isn't working either (it's either 'password' or 'admin' and neither of those work). 

Is there any reason why it would no longer work? I read in the user manual today that the password should have at least one uppercase and one lowercase and my password didn't have an uppercase (but it obviously let me set it 2 years ago) so could it have been a firmware update which somehow invalidates the current password? 

I really don't want to factory reset as they are ceiling mounted so it would be really inconvenient, any suggestions very welcome.