Good news!
As I expected, changing the username of the admin account did not allow me access to the content. While the start menu and logon screen and the like display the changed name, under the hood, Windows still retains the initial username. Since the connection to the SQL database is done via Windows authentication, it was still trying to use my "new" name - hence no access.
Instead, however, by going into the user accounts, it's possible to create a NEW administrative user (I am guessing admin will be required to be able to use the toolset/connect to the database) with the same name as the old user - and then switch to this account - this user can then check everything in. So no need to reinstall Windows again - just create a new user with the right name.
I'm not sure how this would go if the computer name had changed - since that requires a full reboot, that *might* actually change enough things under the hood that it would be fine.
Either way - I can now access, view, and checkout all my resources - and they're all intact! Again, thank you both for your help!
The only weird thing that remains is that it seems that new level layouts that I've made don't display properly in the area editor - all the structure is present, but all the textures and the lighting information are all missing. It seems that the toolset pulls level layout information/appearance from a different place to the game itself - as all the levels look fine in-game.
I wonder if I can copy my level folders from the Documents\BioWare\Dragon Age\packages\core\override\toolsetexport\ to another directory to get the toolset to read them? The alternative seems to be to re-export, and I'd rather not go through the hassle (plus I'm pretty sure that one .erf file got corrupted at some point and really doesn't want to re-export any more)