This was announced awhile ago but MMO Champion is just now reporting it:
Any player who has been on the realm for longer than 60 days is entitled to a realm first achievement. In this particular case, the matter was investigated thoroughly for all parties involved and everyone who was entitled to the achievement should now have received it.
The reason it was changed was to stop players practise on one realm then transfer over to a newer realm and get the achievement, potentially blocking the players on that realm who have been there longer earning it themselves.
I wasn’t aware people were still doing this. I know people were hopping servers to get more Trial of the Grand Crusader attempts, but they already fixed that. 60 days is a long time though, if you think about it. I don’t think I’d qualify yet on Sargeras if we got a realm first. I’m not sure if this was put into place when we got Grand Crusader, because I was driving home from my families house after Thanksgiving weekend the night we did it. :(.