Changes in Oldwick, N.J.-based A.M. Best Co.'s rating system has critics and competitors wondering: Will eight new ratings categories have a genuine effect on the way insurance companies are scrutinized?

Best announced a revised rating structure Monday that will increase the number of categories by which insurance companies are judged, to 15, from nine.The new categories are: A++, B++, C++, D, E and F.

The D, E, and F ratings reflect companies that are "below minimum (Best) requirements," "under state supervision" and "in liquidation," respectively.

These three new categories replace Best's former categories of NA-7 and NA- 10. NA-7 meant ratings were not assigned while NA-10 meant regulatory action was indicated.

"The bottom line is not the specific scale, but their (A.M. Best) track record in alerting customers to failures and helping them to select strong companies," said Martin Weiss, president of West Palm Beach-based Weiss Research Inc.

"There was a higher failure rate of their higher-rated companies than their lower-rated companies in 1990," Mr. Weiss said.

He said that it is Best's prerogative to alter the scale and methodology for rating companies, but given the rating agency's history, he questions whether the changes address the issue of insightful rating.

Bill O'Neal, senior vice president at New York-based Standard and Poor's Insurance Rating Service, said, "The announcement (of the changes) is irrelevant because Best didn't indicate which companies will be downgraded or upgraded under the new system."

What Best did was "give a finer breakdown of our opinion on the financial strength of insurance companies," said Paul Wish, a vice president at Best.

According to Best officials, the new categories are designed to "address the increased awareness of insurance professionals and the public."

But "until the ratings come out in 1992, we will not know if the changes are cosmetic or genuine," said Joseph Belth, publisher of the Insurance Forum, a weekly insurance newsletter.

At the end of 1991, 250 life insurance companies were rated A+ and 40 companies were rated A+ contingent, said Mr. Belth, who also is an insurance professor at Indiana University.

"If under the new system, 250 companies are rated A++ and 40 companies are rated A+, then changes are cosmetic changes because that would mean that they just changed the ratings on the companies without studying the companies," Mr. Belth said.

However, he said, if there is some type of movement in the ratings, then the new ratings could mean that Best reviewed and re-rated the companies.

The only fallout of re-rating insurance companies and possibly retaining an assigned rating would be that "some insurance companies might view the retained rating as a downgrade," Mr. Belth said.

He explained that if a company held an original rating of A+ and the rating is unchanged, then that company and consumers would see the constant rating as a downgrade because A+ is not the highest superior rating and under the new system.

Under the old system, Best used ratings from A+ through C-. Companies that had serious problems, such as those under state conservatorship or in liquidation, were not rated on that system but were given separate ratings on the 10-tier numerical scale.

A rating of NA-11 was added for previously rated companies that have ''experienced a sudden and significant event affecting the company's

financial position," Best officials said.

The rating is only assigned when the financial impact "cannot be evaluated due to a lack of timely or appropriate information."