Dirʿ here means a woman's long garment, not military armor. Mijwal names a shorter garment associated with younger girls. Bayna dirʿin wa-mijwal juxtaposes two dress forms associated with different stages of growth.
Isbakarrat conveys growing tall or entering fuller youthful stature. Reducing it to “changing clothes” misses the interaction between physical growth and the two garments.
Idhā mā introduces a temporal circumstance. The poet now uses dress and stature to suggest a stage of youth, rather than concentrating on the hair, waist, scent, and light that dominated earlier verses.