The Legion pre-patch, 7.0.3, will likely go live on either 7/19 or 7/26. Most of it has already been background-downloaded to your machine in preparation.
Here's a guide
from Blizzard describing some of the changes and the PTR patch notes
. As class changes are a major feature of this patch, click on the individual links within the patch notes to see what changes are in store for your characters.
As you may have heard, they are eliminating a lot of the gold missions at your garrisons in favor of missions that award garrison resources.
One thing that they are doing on the PTR is to automatically fail all of your missions that were still running when you copied your character over to the PTR. Since they may do something similar when you log onto the patch when it first goes live, I wouldn't start any new missions on the Monday before the patch. This isn't a big deal (just wasting resources) for the garrison missions but, by flagging the shipyard missions to all fail, they are exposing your ships to a second round of RNG where they may be destroyed if the original mission chance was less than 90%.
A couple of other garrison changes: The primal spirit vendor in your garrison now sells Felblight for 25 primal spirits. Also the trading post vendor still sells the usual crafting mats, but the prices have gone up by a factor of 4, i.e., the old prices ranged from 4 to 10 garrison resources each, and they now run from 16 to 40 garrison resources. So you may want to stock up before the patch goes live.