I'm hoping this is like when Nami's voice actress "took a break" during Little Garden and the change wasn't noticable. I still rewatch that arc don't notice the difference.
Chopper's, OTOH, was like nails on a chalkboard and as much as I like Kazue Ikura's Koari Makimura (City Hunter/Angel Heart) her Chopper wasn't that great and sounded either too nasally and screechy.
Here's hoping she'll do a good job, but in truth I don't recall Robin having many key lines in this portion of the arc. Then again I recall back in the day when Kae Araki (Miaka from FY, Chibi-Usa from SMR-Stars) had to place Kotono Mitsuishi (Sailor Moon) during the Sailor Moon season finale (where they all died) and she did an excellent job so it's not the first case of Toei having to replace a seiyuu during a critical part of a series, ironically Sailor Moon's original seiyuu returned on the specific episode when she got her SMR brooch power-up.
At least she didn't leave during the "I WANT TO LIVE!!!" scene, that would have sucked.
O-chan