Starting today, 13 stocks on the New York and American stock exchanges, along with their options, will be traded in decimals as the first part of a government-mandated switch-over from fractions. This conversion--the culmination of a five-year push by some members of Congress and regulators--is designed to make it easier for investors to trade and to save money, although there is some disagreement about how large the savings will be. On Sept. 25, the number of NYSE and Amex issues priced in decimals is scheduled to grow to more than 100. The list will include America Online Inc. and Compaq Computer Corp., the two most heavily traded NYSE stocks last year. Nasdaq stocks will continue using fractions for seven more months, as Nasdaq completes the computer upgrades its system needs to handle decimal trading. But all U.S. securities are expected to be converted to decimals by April 9. The Los Angeles Times and other publications have already begun printing stock prices in decimal form.