move the goalposts

The EconomistのFRの政策とそれに伴い他の国で行われている経済政策に関する記事から英語表現を取り上げます。赤字のmove the goalpostsは、推測しやすいと思いますが「規則や契約などが決定した後に、それを覆す」と言う意味で使われるイディオムです。ちなみにアメリカ英語ですと、move the goalsとなります。（Othello）

Flypaper - The markets and monetary policy
EQUITY markets fell on Thursday in response to the minutes of the Federal Reserve January meeting, which suggested that officials had doubts about further quantitative easing. My colleague on Free Exchange criticised the bank's muddled message. There has been a drive over the last couple of decades towards central bank transparency but transparency isn't much help if the message isn't clear. We have moved from the Fed committing to low rates for a given time period, to a commitment to low rates until unemployment falls to 6.5%; are we moving the goalposts again?