Does the "b" in "mbili" change to a "w" no matter what class of noun it is grouped in with?
Well, not all noun classes, only the ones that indicate the plural of a noun (for obvious reasons :) Otherwise, yes, with the exception of the N/N class. This is the list:
II) Watu wawili - Two people
IV) Miti miwili - Two trees
VI) Maembe mawili - Two mangoes
VIII) Viti viwili - Two chairs
X) Nyumba mbili - Two houses
Please check this thread about the adjective concords of the N/N class: https://forum.duolingo.com/comment/30738085