I just changed my whole campaign from broad match to phrase match. I think, now, that was a huge mistake.
It seems as if the google system looks this type of changing as a brand new word. Therefore whatever benefit you gained for a high CTR on a given word(s) is lost.
For example, if you had brown widgets as a broadmatch with a high cllck through and therefore a high ranking, and change to "brown widgets" you lose all of your click through history and any benefit you may have had because of it.
tom