If you perform this upgrade, you may lose the ability to run certain combinations of components. I discovered this the hard way.
On my TT bike, I run basically a 9070 group (9070 FD & RD, R671 TT shifters, modified climbing remote, and internal battery). Due to some routing considerations, I've been running the EW-67 junction box instead of the EW-90 variants just because the form-factor worked best for my routing. All has worked well. When I need to charge the system, I'd plug in a spare EW-90 and do the charging.
I agreed to the upgraded firmware and.... nothing. A fully-charged system no longer shifts at all with the EW-67 as the junction. Shimano USA confirmed that this is the case (I was the first one to report but they tested it).
Some of you out there might be running a 6870 or 9070 set of Di2 parts along with a 6770 (10s) RD, as a way to delay an upgrade to 11s wheels. If you upgrade the battery's firmware, this combination will no longer work. There's no warning that conveys this is what is going to happen, and Shimano does not document the changes in their releases in a meaningful way.
There is also no going back; unless you buy a new battery with the older firmware, you cannot roll back. I tried (installing an old version of the E-Tube software thinking I could possibly force an "upgrade" to an older version--but it is still smart enough to check that the battery has more recent FW, and the older E-Tube software will not run).
There is no fix offered from Shimano.
Of course it is ultimately up to the end user to make a decision to upgrade the firmware, but most will see the blinking button and give it the attention it demands. 9070/6870 + 6770 RD was never an officially supported configuration...so if you run this and want to preserve it, it is best to never upgrade the firmware. Same deal with the EW-67.
Hope this helps someone.