Update is out next Thursday.
They added act 4.
They're removing the cruel difficulty (previously you had to run through acts 1-3 twice consecutively, now you run 1-4 + 3 small "interlude acts" that serve as fodder to level you up)
They added new tiers to support gems and removed the restriction that stopped you from having multiple of the same type supporting different skills.
They added a new auction house system that allows trading buying items from other players with no need for them to accept the trade, so if an item is listed on the trade site you may always buy it instantly.
There's a lot of balance changes and additions to the crafting system, too. It's looking like a biiiiig patch.
time to line up for the slop!
No, you can do it any time. It's just that the tabs themselves have to have been purchased before then.
You do lose the storage space, though, that is true. I think this doesn't really matter unless you play both games and 3.27 doesn't get the new system? Because otherwise, if you only play POE1, just wait until they add the system to POE1 before you change your tabs to merchant tabs, which will probably be next league anyway.