Mz Migraine, what I was told by the woman who helped me at my local SS office with my claim is a little different than what you posted, so I looked it up on the SS website.

now I am not sure if she told me correctly. She told me that at retirement, the SSDI converts over to SS, and the amount remains the same. When I read this web page, it sort of sounds like it depends on the year you were born - which determines "Full Retirement Age." I was born in 1958, so I would be full retirement age at 66 and 8 months. Now, if they threaten to take away SSDI before then, say at age 62, and I opt for SS, my benefit would be reduced by 30%.

At least if I understand this correctly....

http://www.ssa.gov/dibplan/dqualify.htm