Aug 10 - Standard & Poor's Ratings Services said today that its 'BB-' rating and stable outlook on Baton Rouge, La.-based outdoor advertising operator Lamar Advertising Co. and its operating subsidiary, Lamar Media Corp. (Lamar), are not immediately affected by the company's announcement that it's considering converting into a real estate investment trust (REIT). Under a REIT structure, the company would be required to distribute 90% of taxable income to shareholders, which in our opinion could