American Express Scrambles to Remake Its Credit Unit
American Express Co, scrambling to regain market share, has shifted focus from charge-card business to more popular credit cards, introducing variety of new cards in last 18 months; has signed up hundreds of thousands of new merchants and reversed decline in number of card holders, sharply increased loan balances and raised overall profitability of its cards; gains have not kept pace with those of leading card issuer; vice chairman Kenneth I Chenault cites plans to speed growth; will let banks issue own American Express cards; table; drawing; photos (M)