I think it could be September or maybe even November.
Part of me thinks November because that's when the next sweeps period will be. If you're going to use HD as a promotional tool to drive viewership and spike the ratings, you might as well do it right at the beginning of the book. Capitalize on the novelty of being the latest, greatest HD in the market. Even people who don't normally watch WROC will be bound to tune in just to see what all the hype is about, especially if WROC manages to somehow outdo WHAM's set. If WROC does it well, they might win some people over. If they don't, they're going to lose people as the novelty wears off. Might as well capture those "novelty" viewers (and thus, take them away from their usual favorite) during the book, when it counts the most. Picking up those extra viewers for a just few days in September won't matter because Nielsen isn't measuring local TV in Rochester in September.
On the other hand, an HD launch in September to coincide with new "fall season" will grab the attention of your "I only watch the news when there's a good lead-in" segment of the audience, people who probably haven't watched much TV since many shows had their finales in May. In this case, WROC would be relying on using the early part of the fall to build a solid audience during the weeks leading up to sweeps. The other pro here is that, if there are any unforseen HD problems (the kind that NEVER arise during off-air rehearsals, only when they actually go live on-air for the first time), those things are happening outside of the book. That way, when the book does begin, everything's hopefully running smoothly and all the kinks are worked out.
I'd also side with September if they're already hinting at things in August. There's no reason to tease viewers for more than two whole months, that's just too much hype for what it is. Maybe if they were going to be first, but they aren't. Another year or two and we'll see the SD holdouts flipping to HD quietly, so as NOT to draw attention to the fact they took so long to catch up with every other channel on television. I mean seriously, whenever WHEC finally catches up, how much "oomph" can their promos have?
"News10NBC in HD... let's all finally be there!"