When you are done the main quests in Broken Valley (about level 15) and complete another area you will be able to transform into a dragon at will (about level 17 up to about 24, if you do a bunch of other stuff first), assuming there is enough room and you are not within an anti-dragon shield. There is a short sequence where you get to fly around as a dragon about level 6-8, at a main plot event.
However, you can not attack ground opponents in dragon form. The game was originally designed to allow that, but it required too much memory and had too much of a frame rate hit; see Lar's post in the topic
Design ideas for Divinity 3 (mostly for Larian).
You can sometimes attack flying opponents in human form, though.
The comment you read may have been referring to the add-on section of the game. It was originally going to include a Broken Valley type area, but that got cut due to resource limits. Larian decided that overall the game needed more of a human hub of activity, so focused on that, leaving just one dragon section at the end of FoV.
DKS does not support SLI, though most people have not had a problem with SLI/Crossfire systems.
The graphics and performance should be better in DKS, though load times are a little longer (may or may not be noticeable, depending on your system).
The controls are pretty much the same, though there were changes to the UI.
The flying fortresses were redesigned, and most anti-dragon shields were removed.
Quest experience was adjusted, to make leveling easier at the start of the game, and difficulty spikes in general were addressed.
The level dependence in combat was reduced, so opponents more than 3 or 4 levels above you are not invincible, and those 3 or 4 levels below you can still sometimes be dangerous in groups.
Enemy placement was also changed, especially at the start of the fjords.
The equipment sets that were in D2:ED are in different locations, and more sets were added. Merchants can now carry very good pre-determined items (unique or set items) in addition to random items.
Stat requirements were removed from equipment, so now you only need to meet the level requirement.
Some dragon skill books have been moved or removed from the D2:ED locations (the librarian in Maxos temple no longer gives you one, despite still mentioning it in the conversation).
The effects and level requirements of some enchantments have been changed (generally higher effects and lower requirements), which are also used by the game's random equipment generator.
In DKS magic damage bonuses on weapons are applied to spells, as well (including each magic missile in a cast of the spell), so mages should duel wield for best effect (no need to boost the associated skill, as the off-hand penalty is not applied to spells).
