The biggest issue is with people who have one password saved and now the site wants them to use "the right one".
Do they know which one goes where? (how do they know?)
Do they know where they have them stored?
A good thing they did is allow the pdf download but it offers them no assistance in why or which key.
The first impact I can predict will show if users are struggling is a reduction in wallet transactions. Depending on what key they have saved they should have access to either the wallet or the site. They likely will not know which one to use where.
Testing generally involves testing some various levels of users to ensure that changes are intuitive and people have the ability to either find help or troubleshoot.
Again, I'm not angry or trying to be rude. I don't think anyone, even those of us who looked at it expected it to go live in the middle of the day on a Wednesday without any preparation.
It will sort out and is not the end of the world, but again, a bit messy.