Shares of Blockbuster soared 32 percent Wednesday after it announced that it had reduced the amount of letters of credit to Viacom, its former owner, to $25 million from $75 million. It had previously cut the amount by $25 million from the original $100 million. It had previously announced that it had sold an Irish subsidiary, Extra-Vision Ltd., for $45 million to Ireland-based Birchhall Investments. (At midday today, Blockbuster shares had retreated 8.4 percent.)