NL Industries Inc., Houston, may make investment manager and portfolio changes to its 401(k) fund, which now uses four managers and 13 investment options. "We will probably make changes, said John St. Wrba, assistant treasurer. He is doing a major study in-house of the portfolio choices and performance, which he expects to complete by the end of March and then decide on changes. The $25 million fund offers seven investment choices managed by Merrill Lynch; two portfolios by AIM; two by MFS; and one by Templeton. It also offers a company stock option.