While I was at Wicked, an official source at the Gershwin (he/she would like to remain unnamed) confirmed to me that Kate Fahrner and Donna Vivino will be the next pair of Glinda/Elphaba standbys. The date for Donna's takeover will be next Tuesday the 23 of August. Kate's is the same.
I actually happened to see Donna coming out of the Gershwin, and talked to her and took a picture with her :) She is so very nice!
Another source has also confirmed to me that Jenny Fellner will be returning to the role of Nessarose at a later date this year :)
Thanks, and I will have the review posted soon :)