I mainly do not like the switching of the armor class during rests, that is plain useless to me. Either a character is optimized on stealth or it is not. That is like giving a rogue heavy armor proficiency. It does not give him anything, because heavy armor interferes with his assumed dex bonus.
I hope they rule that one different, it is outright bad design.
I'm leaning both ways on this, which makes me very dizzy.
On one hand, back in the day, a warforged had to spend a feat at first level to get anything other than the basic composite plating (+2 AC, the equivalent of leather armor in 3e, counts as light armor). The Eberron setting book had two feats: mithral plating (IIRC equal to a mithral breastplate, counts as light armor but protects as medium) or adamantine plating (equivalent of full plate + DR 3/adamantine). Races of Eberron added two more for specialized purposes: unarmored (remove the basic armor but count as unarmored, useful for monks) and darkwood (replace metal components with super-hard wooden ones, for warforged who want to be druids). Anyhow, my point was that this feat had to be taken at first level and could not then be changed.
On the other hand, 3e warforged were clearly able to gradually change their composition. There were feats to improve various aspects (e.g. more DR for adamantine, I think lower armor check penalty/higher maxDex for mithral), and one prestige class in the core book (Juggernaut, for leaning strongly into the construct side of "living construct", growing spikes and getting more DR and crit immunity) and another one in Races of Eberron (don't recall the name, but it went the other way, becoming more living). They could also get their core armor improved just like other magic armor.
On the third limb (I guess I'm doing feet now), these improvements felt rather slow to me. A warforged doesn't become a juggernaut overnight, it takes them however long time it takes to level up.
I guess that where I eventually land (on the other foot, then) is that I'm cool with the concept of warforged changing their integrated protection, but on a slower time-scale than a long rest. Perhaps when leveling up, or as a downtime activity taking a week or so. That would allow for a warforged to e.g discover a taste for the wild and switching to a lighter model when taking ranger levels, or something like that.