Too good to be true

I suspect "it looks too good to be true" is an old phrase but just when did it get amended to "if it looks too good to be true, it probably is"?

The Oxford English Dictionary has found the phrase "too good to be true" from 1580. I don't know when it became, as you say, amended.
SS

I'd guess 1581, since the word "too" makes the idea clear. Amending the explanation is just redundant.