Define what you mean by "risk"
I have yet to see what evidence applies in this case to the level of risk (either regulated by statue or to somebody's concept of safety).
Makes sense
safety is a state of mind prediction of the future based on the data at hand.
Just because something has broken doesn't mean that it is now unsafe.
You really believe in the 'I don't know anything about it, so it doesn't exist' approach to safety, don't you?
Fortunately the FAA (who have a clear definition of risk, BTW) is going to have to persuade the Japanese and European's - as well as Congress- with scientific method not bull****.
but something tells me retrofit may be getting serious consideration
Airbus stated it would take them 3-4 months to change the battery in the a350, its not something they would consider doing if they didn't have to.
Identifying what went wrong with the battery will probably take a long time. Even if its a manufacturing fault you will have to determine that the fault could not occur again and you can detect it. It was already supposed to be the most reliable technology available.