SO
Walpurgis rising takes place after rebellion, assumably in the new reality Homura made. BECAUSE the reality was changed drastically that could’ve altered everyone’s souls including the Holy Quintet’s (it seems that way to me as after the reality has been changed/remade in rebellion, everyone‘s personalities feel a bit different. aside from Sayaka). And in a lot of PMMM media, it appears your soul is what decides your magical girl outfit as a lot of magical girl’s outfits reflect their soul/identity/personality etc, and because everyone’s souls might’ve been tweaked that might be why everyone’s magical outfits got redesigns.
And for Madoka having the biggest redesign in her magical girl form can connect to the fact that Madoka in Homura’s reality is only one half of True madoka’s soul. And because she isn’t complete, that could be the reason why her redesigned magical form is so different from the ones before Walpurgis rising. Same for Homura, since her soul is altered due to having some of Madokami’s power from splitting her soul into two parts and gaining a new soulgem + becoming a god as well, since in my opinion Homura has the 2nd most noticeable redesign.