how did you "replace" it? You may have to parse all children/surfaces/brushes and "BrushTexture" the right brush layer with the new lightmap texture. This may be a simple grey quad in rgb 127,127,127.
if you want to use directX lights, you may also have to alter the EntityFX, eg. from 1 to 0.
if you want to use directX lights, you may also have to alter the EntityFX, eg. from 1 to 0.