“Moving the goal posts” (“Moving the goalposts”)

“Moving the goalposts” (or “moving the goal posts”) is a sports analogy, similar to “raising the bar” or “lowering the bar.” If the goalposts are moved in, it’s easier for a football kicker to score; if the goalposts are moved out, it’s harder to score.
 
The phrase was popular in the 1980s, but there are some 1970s citations, including an economic one from 1972.
 
   
The Phrase Finder
Moving the goalposts
Meaning

Changing the target of a process or competition to by one side in order to gain advantage.
 
Origin
This phrase is a straightforward derivation from sports that use goalposts, i.e. Football, Rugby Football, American Football etc. The figurative use alludes to the perceived unfairness in changing the goal one is trying to achieve after the process one is engaged in has already started.
 
The phrase came into wide use in the UK during the 1980s.
 
Wikipedia: Moving the goalpost
Moving the goalpost, also known as raising the bar, is an informal logically fallacious argument in which evidence presented in response to a specific claim is dismissed and some other (often greater) evidence is demanded. In other words, after an attempt has been made to score a goal, the goalposts are moved to exclude the attempt. This attempts to leave the impression that an argument had a fair hearing while actually reaching a preordained conclusion. Moving the goalpost can also take the form of reverse feature creep, in which features are eliminated from a product, and the goal of the project is redefined in such a way as to exclude the eliminated features.
   
Time magazine
Moving the Goal Posts
Monday, Mar. 06, 1972
TO economists and politicians, “full employment” does not mean what the words suggest: a job for absolutely everybody who wants one. Instead, the working definition has long been a jobless rate no higher than 4%. Even by that measure, the U.S. has rarely enjoyed full employment since World War II; the last time was in the closing months of the Johnson Administration and the early days of the Nixon era. Now the President’s aides are redoubling efforts to bring the jobless rate back from nearly 6% toward full employment by the elections. Instead of launching another new economic game plan, however, they are trying to move the goal posts.
 
5 March 1977, Victoria (TX) Advocate, “Plan Unveiled for Future of Coast,” pg. 14A, col. 4:
He gave this as industry’s position: “We don’t mind kicking at the goal posts, an we don’t mind being moved back some, but don’t keep moving the goal posts, the way the federal government does.”
 
4 February 1978, Washington (DC) Post, pg. C12:
“They keep moving the goal posts,” he lamented.  “We’re not afraid of deregulation, though,” he said, “if they really took off all the wraps.”
(Albert V. Casey, chairman and president of American Airlines—ed.)
 
Google News Archive
2 October 1979, Spokesman-Review (Spokane, WA), “EPA ratings can deceive,” pg. 4, col. 2:
EPA has promised to try to do better. The DOT could adjust the standards upwards, as has been proposed, so as to account for the shortfall, but this would penalize those car builders who have invested heavily in trying to meet current fuel economy levels. Moving the goal posts is frowned on.
   
Google Books
Turning the Tide : a new policy for Canada’s Pacific fisheries : final report
By Peter H Pearse; Commission on Pacific Fisheries Policy (Canada)
Vancouver, BC: The Commission
1982
Pg. 4:
It is one thing to change the rules of the game and it is quite another to keep moving the goal posts.