Assuming that the statement "all armies will be playable" includes all their options and what not, as given in the current Codex line, expansions, supplements, and dataslates, FAQs, errata, and addenda, I guess no army, for now, has to change at all.
If the blanket statement turns out to be true, then Death Company Blood Angels marines still have the option to all take a power weapon, and to choose between a jump pack and a regular backpack, for example. Likewise, Imperial Astra Guard Militarum will have Sergeants in their basic squads that cannot take a standard lasgun with them, and they can include a heavy weapons team, even if the rules for them might differ wildly from what we have now. Hormagaunts will come in broods of 10 to 30 models inclusive. I can go on, but you probably get my drift. If the blanket statement is true, nothing changes, except perhaps a couple of point values, and the way some unit play on the battlefield due to their rules having been changed (Space marine Devastators becoming more mobile due to scrapping of snapshots and only applying -1BS after moving, for example).
On the other hand, assumptions are the mother of all ... things that can go wrong, and there things might go wrong with current armies. Mayhap small things. If all of a sudden Death Company Marines are limited to 10 men to a squad, instead of the current 15, that means I have three full squads instead of two. If those same Death Company Marines are limited to one squad of five Marines, I have 25 DC Marines to spare. Little difference, big difference. The same goes for any significant changes in, for example, Ork Boyz Mobs. If they are suddenly changed to be mobs of 10 or 12 boyz at maximum, because that's all that fits in a Trukk, huge infantry blobs of green, mean fightin' Boyz are history as well. Depending on how much the army composition and even individual options change. That "Army will still be playable", just not in the way or composition I am used to. The blanket statement is then still as true as ever. Some of these examples will change the army, however.
In a way, the coming of the new edition is both a blessing and a curse. I have started a new army with every (truly) new edition. Seems like a little wait, and my plans for a new Ultramarines force (yes, I have always just wondered when to start one) might come to fruition. I already have Robby G for them, and in this way, with the start of his Crusade, adding the NuMarines doesn't seem like a stretch either.
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