Last summer Sears introduced a credit card for its best customers. Now the upgraded card has gotten better. Sears, Roebuck and Co. introduced this week an enhanced benefits program for its Sears Premier Card customers.  Under the new program, customers will be able to earn Sears Premier Reward Certificate coupons that can be used toward Sears merchandise and services.  The coupons will be equivalent to 1 to 3 percent of purchase dollars, with greater reward levels for higher purchasing activity. Since the introduction of the Sears Premier Card in July of last year, its customers have been offered priority in-home service calls, special preview shopping days prior to key sales events and exclusive money-saving offers in addition to all the benefits of the Sears Card.  The Sears Premier Rewards program replaces the recently discontinued Sears Bonus Club, which issued cash value coupons to customers based on their Sears Card purchases. The Sears Premier Card has been issued to 13 million Sears Card customers since July 1999.  Based on a semiannual review of account activity, Sears Card customers are upgraded to the Sears Premier Card when they have made Sears Card purchases totaling $600 or more in the previous 12 months. For more information visit www.sears.com.