Setting the GL_NEAREST/GL_LINEAR filter parameter on a texture can be done without re-uploading the texture's image from scratch, however this ability is not available through the QGLTexture2D API. This would be very useful so that for situations where changing between GL_NEAREST and GL_LINEAR is necessary or desirable, this change can be made without uploading two textures or keeping the original image data around and re-uploading (which comes at a sometimes significant time cost).

http://codereview.qt-project.org/#change,21497 contains a suggested patch to implement this ability.