I suspect many of us that use the Preview Grid have avoided changing passwords because changing complicates our access to the Preview Grid (ADITI). I’ve changed mine and filed a trouble ticket to get my ADITI password sync’d up with my AGNI (main grid) password.
Others are running into the problem. I guess it is enough people that the Lindens decided it was time to do something. See: Some may experience login problems to the Beta grid. We are on it. 12/15/2015.