I don't think this is correct. Planet Money did a recent podcast on this very subject (Would A Downgrade Matter?)[1], and they concluded that a downgrade from AAA to AA+ doesn't matter very much in the long run. Yes, it's somewhat embarrassing, and interest rates are likely to go up _slightly_, but that's about it.
The big leap is from "investment grade" securities to "junk bonds" ('BB'/'Ba' or less). We're still a long way from there.