D6i changes have always been limited to specifically trained and equipped service facilities, not just every dealer. In the US, the D6i can be serviced by a "Level One" facility. Training and equipping is limited to those selected/approved by the Huish rep, and is a bit on the pricey side. So it is not a simple matter for all LDS to be able to do it.
I will also note that aside from changing the battery, that service center is also trained and equipped for the bi-annual service on the control buttons for the wrist top computers. Bet you didn't know that was a thing, right?
Now, here is the kicker - I used to be able to change the batteries on the transmitters as a dealer. But since the recall last year, those are now limited to the Service Centers for battery changes.
If you had a shop that was changing the D6i battery in the past without the training, that was a non-authorized service. It can be argued all day long whether that is good or bad, but I am almost certain that would mean you were long overdue on getting your buttons serviced, running a risk of non-battery related problems.