As of today, Games Workshop has now updated their Apocalypse mini-site to include data sheets for all of the factions and these are available to download for free.
A cursory glance at these shows the changes made in order to suit the scaled-down system which Apocalypse will feature. For example, Guilliman only has 2 attacks and 2 wounds in Apocalypse as opposed to his mighty 6 attacks and 9 wounds in standard 40k.
Some more things to point out at this early stage are differences in Power Level when compared to the 40k version of the model (the Land Raider Crusader is Power Level 16 in 40k and Power Level 19 in Apocalypse) and some changes/limitations in load outs. Again using the Land Raider Crusader as an example, in 40k you can equip the vehicle with a Storm Bolter and/or a Hunter-Killer Missile, but these options are missing from the Apocalypse Data Sheet.
The Apocalypse Data Sheets are available to download in full or by faction on the Games Workshop sub-site: https://warhammer40000.com/apocalypse/datasheets/