Update: I favor Julia's theory in the comments below that this move is really about keeping Cold Case in primetime on a night where CBS anticipates some schedule delays due to overrun from having the national broadcast of the late afternoon NFL game on Sunday.
James Hibberd reports that CBS will be doing an experiment next Sunday moving Cold Case to 9pm and Three Rivers to 10pm to see if it improves either or both shows' ratings.
Hibberd also reports that CBS is still considering both ordering new scripts for Three Rivers, as well as ordering new episodes (5) and could be waiting to see the finals from Sunday before making a decision after Three Rivers hit season highs in the preliminary numbers.
Clearly CBS is attempting to learn something. But whether it is that Cold Case's ratings are or are not due to the Three Rivers lead-in or merely finding out whether Cold Case fares better against Desperate Housewives than Three Rivers, potentially improving both, or whether CBS just wants to be able to say "hey, see, we tried Three Rivers when it wasn't against Desperate Housewives so don't say we didn't try..." (update: Desperate Housewives isn't even on next week. ABC will be airing the "American Music Awards").