(Rewrites, adds legal expert comment, updates shares) May 14 (Reuters) - CBS Corp filed a lawsuit on Monday to reduce the voting power of controlling shareholder National Amusements Inc, the movie theater company owned by Sumner and Shari Redstone, in an act of defiance aimed at thwarting the Redstones' plan to merge CBS with Viacom Inc. CBS said in its lawsuit it is seeking to prevent the Redstones from ousting any directors or changing the company's bylaws before its full board meets on Thursday to consider issuing a dividend that would curb National Amusements' voting power. If the dividend is approved, National Amusements' voting rights in CBS would shrink to about 17 percent from about 80 percent, more in line with CBS's economic stake of 10.3 percent, according to CBS's legal complaint, filed with the Court of Chancery in Delaware.