The Legion Pre-Launch will occur some weeks before the expansion hits on 8/30. The pre-launch will contain all the class changes and it will affect everyone, not just those that have pre-purchased Legion.

The pre-launch (7.0.3) is currently up on the PTR for testing and you may want to try it out, especially if you have a lot of alts. Or if you have a survival hunter or combat rogue as those specs were completely changed and you may want to consider changing specs on those characters. Since all of your character's talents will be reset, it will be necessary to select new talents and repopulate your action bars. To test the changes, rather than trying things out on training dummies, you can participate in the Legion Invasion Scenarios. These award Nethershards and chests containing level 700 gear. When the patch goes live, you may want to run these to acquire the Nethershards and the level 700 weapons. For 150 Nethershards you can buy Coalesced Fel which can upgrade these weapons by 5 levels up to a maximum of level 725.

Once you have your characters tuned on the PTR, be sure to take a screen shot showing your talent tree and action bars so that you can reapply the changes once the pre-Launch goes live. You can't count on the PTR remaining up when the patch goes live and they have also been known to do a character wipe on the PTR.