The Recording Academy makes changes to its rule book every year, probably more so to best new artist than any other category.
The goal is to strike the right balance -- not too restrictive (Richard Marx was disqualified in 1987 for having contributed one track to a soundtrack album the previous year) or too permissive (Shelby Lynne won when she was on her sixth album).
Largely as a result of these rule changes, six artists who won best new artist in the past wouldn't be eligible to compete in that category today.
Here's a complete list: 1962 - Robert Goulet: The star had appeared on the Broadway cast album to Camelot, which was nominated for the 1960 award for best show album (original cast), now known as best musical theater album.
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