Only data I’ve seen is this.
My guess is that it might be November as that syncs up with the 90 day timeframe to consider opening transfers to/from the new servers to be launched 30 August with the pre-patch. Also bringing y’all in later brings another wave of lower leveled characters to aid with the running of sub-80 dungeons to keep that activity level higher.
Hope they decide to provide more data soon but they seem to be waiting and giving just a few weeks notice with a lot of other decisions.