If it ain't broke, don't fix it.
if a system or method works well there is no reason to change it.
"We're happy with our exam system in Scotland, and as they say, if it ain't broke, don't fix it."
"He hasn't made a lot of changes to the team since taking over as head coach, figuring if it ain't broke, don't fix it."
(Cambridge Idioms Dictionary, 2nd ed. )

IF IT AIN'T BROKE, DON'T FIX IT - "Any attempt to improve on a system that already works is pointless and may even be detrimental. Originated in the United States in the twentieth century. Government official Bert Lance (1931- ) was quoted in the May 1977 issue of 'Nation's Business' as saying, 'If it ain't broke don't fix it.' Lance's advice, according to William Safire, 'has become a source of inspiration to anti-activists.'." From "Random House Dictionary of Popular Proverbs and Sayings" (1996) by Gregory Y. Titelman (Random House, New York, 1996).
http://is.gd/d2WIX

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Note added at 25 mins (2010-06-25 03:24:44 GMT)
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Estou mais acostumado a "em time que está ganhando não se mexe" :)