Three great replies, thanks! Right out of Goldilocks and the Three Bears, too.
To Hilary's question, correct, it comes down to voice and tone guidelines in the end, but we're presenting them as personas because we find that creating a persona based on our understanding of the target audience makes it easier for those stakeholders to understand and trust our recommendations. Some of them are a little skeptical about the value of this project :)
I'm leaning toward a just-do-it-all-at-once approach as I don't think it will be a dramatic shift. It's a corporate client so we're not going off the deep end here, just a number of smaller shifts to engage the audience a bit better. Looking for any hard data to back up that recommendation, but I bet that's going to be hard to come by.