What may be the symbolic meanings of those 9 forms of Mother Goddess Durga ?

Very nicely expressed! Yes, indeed! From mere 'annamaya-kosa' and 'praanamaya-kosa', through the great 'jnanamaya-kosa' to the 'aanandamaya-kosa', the life cycle progresses, the male energy contributing duly to complement the female creative force, thus sustaining the 'mission' , the preservation or 'sthithi' ! Or, another angle: the 'ashtaanga': from yama, niyama through aasana and praanayaama, on to pratyaahara and thence dharana, dhyana and samaadhi which final stage is the "Siddhi"! (ashtaanga symbolically obtaining from the stage of child, progressively in steps, leading to the final attainment, "blessing old mother" aas you have so nicely put it!