Hoi, When Wikipedia moves to CC-by-sa, it means that all the Wikipedia articles go to the new license.
It does not necessarily make a difference to the pictures and other material in Commons as long as the criteria for inclusion in Commons stay the same. This means that the GFDL will continue to be a valid license for new material. When people use a Wikipedia article and an accompanying picture is GFDL then there is basically no issue as it is only when the picture is lifted from the article that a problem may occur.
With the FSF, the CC and the WMF are agreeing that the GFDL and the CC-by-sa are compatible, it means that it can be argued that the inclusion of GFDL material does not negate the provisions of the included material, it is compatible and the material can be used in a CC-by-sa work. This means to me that the GFDL material does not lose its license and when material is used outside of the "compatibility box" it needs to be used in full compliance with the GFDL license.
As we are in the process of understanding what it means that we move Wikipedia to another license, it is important to address what it means for the material in Commons that is explicitly labelled GFDL and the future material that will be explicitly labelled GFDL. This needs to be addressed because the alternative would to be that we will at some moment no longer accept GFDL material and that is to me not a good thing.
Thanks, GerardM