The results promise to be tangible. NOAA believes there will be a "significant impact" to one- and two-day forceasts, and improve the overall accuracy for forecasts up to a week ahead. It also hopes for further improvements to both the physics as well as the system that ingests data and invokes the weather model. This is on top of previous upgrades to NOAA supercomputers that should provide more capacity.
The old model will still hang around through September, although not as a backup -- it's strictly there for data access and performance comparisons. FV3 had been chosen years ago to replace the old core, and it's been in parallel testing for over a year.
Not everyone is completely satisfied with the new model. Ars Technica pointed out that the weather community is concerned about surface temperatures that have skewed low, for instance. It should be more accurate overall, though, and that could be crucial for tracking hurricanes, blizzards and other serious weather patterns that can evolve by the hour.