Script to migrate FSS to blob. You have to write an uprade step and put this code inside. Most of the time you need to adapt it to your project / content types

removefss.py

def upgrade_fss2attr_storage(portal_setup):
    """We use fss on plone3, so we need first to come back to default
    plone so the migration of plone will next do the job to go on blob"""
    site = portal_setup.aq_parent
    #fss2attributestorage
    from cStringIO import StringIO 
    from Products.Archetypes.Storage import AttributeStorage 
    from iw.fss.FileSystemStorage import FileSystemStorage 
#    attr_storage = AttributeStorage()
    fss_storage = FileSystemStorage()
    catalog = getToolByName(site, 'portal_catalog')

    brains = catalog(portal_type="File")
    files = [b.getObject() for b in brains]

    brains = catalog(portal_type="Image")
    images = [b.getObject() for b in brains]

    def migrateFile(f): 
        field = f.Schema()['file']
        content = StringIO(str(fss_storage.get('file', f)))
        fss_storage.unset('file', f)
        field.set(f, content)

    def migrateImage(img):
        field = img.Schema()['image']
        try:
            content = StringIO(str(fss_storage.get('image', img)))
        except AttributeError:
            logger.error('img %s has no image attribute'%img)
            return
        fss_storage.unset('image', img)
        field.set(img, content)

    for f in files:
        migrateFile(f)

    for img in images:
        migrateImage(img)