The texture's name or what texture is applied has nothing to do with the model's actual UV space coordinates (other than specifying how a texture is mapped onto the object). In TS itself the only thing that will alter the UV space coords is actually remapping the UV space.
Short version: The UV mapping tells WHERE to put textures. The Texture itself says WHAT to 'paint' the model with.
If you wish to change the texture applied in Truespace, first "Inspect" the area you wish to change the texture on. Then right click on the Texture Map icon in the Shader Box that pops up. This will bring up the Texture Map box with the name of the current texture. Click the Texture Name, select your new texture. Then change to "Paint Over Existing Material" and click on the area of the model you want to change again.
Doing the "Inspect" first insures that all the attributes (roughness, transparency, etc.) that were originally there remain unchanged and only the texture is changed.
You can also change the name of the texture applied directly in the POF model by using PCS. It has a texture editor built in allowing you to change texture names. Note however that this will change all instances of the texture (you can't change the texture for a selected area, all areas that used the old texture would use the new one in this case).
