Long time Fallout fans have feared that Bethesda might retcon the lore in order to put Brotherhood of Steel mentions into Fallout 76 and it has come true. Bethesda posted a tweet describing how the first Elder made contact with Appalachia.
Fallout fans have always been adamant about preserving the series' lore but recent tweets by Pete Hines, as well as a note found in Fallout 76 startled the fans due to a possible retcon. Speculation has been going on for a while, but now it's official - Brotherhood of Steel have indeed been to Appalachia within the first 25 years since the Great War.
Considering that Roger Maxson started the outfit on the West Coast and due to somewhat diminished means of transport, as well as resources, it would be hard to believe anyone would have a reason to go from West to East coast with "just because" being the reason.
Fallout 76 will not be a story-heavy game, since the focus has shifted to gameplay aspects apparently. There will be no human NPCs to interact with and the story will likely be minimalistic. This is another reason why fans may be unhappy about retconning the lore, as there is no sense in doing so for a game that barely has any storytelling.
Another bit of a mess Bethesda made with the retcon is the picture from the official tweet. Brotherhood of Steel designated their West and East coast in different manners. The large gear on the logo was on the left side for the West coast Brotherhood while the East coast had the gear on the right side of the sword. Bethesda clearly pictured a logo of the East coast chapter, that was not supposed to exist for another 150 years.
This particular chapter was founded in 2254, while Fallout 76 takes place in 2102. Fallout 76 is due for release on 23 October 2018 and may contain additional notes that could maybe explain what is going on with the lore. One can't help but wonder if this controversy will be the start of Loregate.