Subtextures and MipMapping

Hello guys,
Do I have to update a mipmapped texture (calling again gluBuild2DMipmaps) when I replace part of it by a subtexture or is it done automatically by OpenGL?
Many thanks,

An update to a mipmap-level will affect that level only. You need to update all levels yourself, unless you use the extension SGIS_generate_mipmap , which will update all levels automatically.