I've shared this video on my Youtube already a couple of days ago, and from the feedback I've heard I thought it might be a good idea to share it here as well. I was trying out the Kanai's Cube a little bit on the PTR, and I decided to show my results and explain the two different paths of item progression that open up with it in patch 2.3. In this video, you can see what the process of Upgrading Yellows to legendaries looks like and how it compares to the Reforge Legendary recipe, backed up with some math to outline the advantages and disadvantages of both.
I crafted 400 yellows bows and upgraded them to legendaries via the Cube, with the goal of getting a good ancient Yang's Recurve. Fortunately, you can craft both the yellow items for upgrading and the blue items to transform the materials in case you have too few of those (for the level 70 yellow version, you need 15 whites/blue/yellows, for the lvl 1 blue belt you need 2 whites/blues). Out of those 400 legendary bows, I got 57 Yang's Recurves (14.25%, 7x ancient), and it took 10000 Death's Breath + 20000 whites/blues/yellows (and another 6000 of each for crafting the yellow bows, but that's unnecessary to do on live when you can just collect them).
TL;DW: Both recipes are okay to use if you really need that one specific item, most importantly build-defining pieces like an ancient weapon. You can farm up most of the materials passively when playing the game, and fortunately they don't overlap - Upgrade Rare requires 25 Death's Breath + 50 white / blue / yellow crafting materials, Reforge Legendary requires 5 of each bounty material and 50 Forgotten Souls. I tried to approximate how many hours of game time you would need to invest to (given the numbers you could reach in high-end parties), and interestingly they turned out to be fairly even. Upgrade Rare seems a little bit more efficient if you have very common items (like Yang's Recurve), and a lot worse for very uncommon items (like Kridershot), while Reforge Legendary always has the same chance to yield the item you seek. All in all, it seems like Upgrade Rare will be much better to use early on in the gearing process on items with several good options (e.g. if you need gloves for 3 different sets, or two different bows), and Reforge Legendary will be used a lot more to target farm very rare items later on.
All of this matches the general progression strategy for season 4 quite nicely, where you would want to do most of the torment farming more towards the beginning and most of the greater rift farming towards the end of the season, simply because focusing on items first makes a lot more sense until paragon grinding becomes more valuable. Both options are fairly well balanced in that regard, and will make it much easier to gear out your main or alternative characters and even the playing field for the leaderboards as a result.
Hope it helps for those trying to gear up quickly after the season starts, or for those that have been hoarding lots of materials on nonseason already!
Would I be able to reforge an old obsidian ring of the zodiac (non-season, no CDR effect) into the new version? I didn't play this season so I never got a new one, and it would be nice if I didn't have to wait for one to drop naturally. I'm guessing blizzard is too clever to allow this, but I'd like to know for sure if someone's already tested it.