You are correct. The season transition and introduction of new PvP items was different this time around, and we apologize for the lack of advanced warning.
The Season 9 set was removed today. The items available for purchase with Honor Points are now considered lower tier Season 10 items. They are still the same items from Season 9 in terms of aesthetics, but the item level and stats are slightly higher to ensure that the Season 10 honor gear has the correct item level relative to the Season 10 conquest gear.
It's likely this is how things will work going forward and we'll be sure to make that more clear when the next season transition takes place.
New basic transition flow:
1) Season X ends and rated play is unavailable; Season X gear becomes available for purchase with HP; CP is wiped.
2) One week later Season Y starts and rated play is available; Season X gear is removed entirely; Season Y introduces a low tier of items which replace Season X vendor items and are available for HP; Season Y introduces the new top items available for CP and rated play. [source]
If it is not entirely clear from the above, or the 51-page post-capped thread, the basic gist is anyone who bought S9 epic gear in the last week got punked by Blizzard. The 365 gear that previously required Conquest Points, then required only Honor points - this is what you would expect in an universe of forms and logic and elegant game design. In bizarro-Blizzard C Team Land, taking the 365 epics and obsoleting them one week later with strictly better in every conceivable way 371 gear, for the same Honor costs even, makes sense. Here is a visual:
On the right, the 365 PvP chest that was 2200 Honor as of June 28th (gemmed and enchanted as you'll notice). On the left, the 371 PvP chest that is 2200 Honor as of July 5th, about 168 hours later. Luckily enough, I had not played my warrior all that often after purchasing the chest, so I still had time remaining to sell it back to the vendor for a 2200 Honor refund, and then buy the new 371 version. Whether the stat upgrade seems like a big deal to you or not really depends on how much you PvP, but one way of looking at it is that the 371 pieces give you 6% more stats.
And, you know, the whole fact this was such a huge designer "Gotcha!" moment; a fitting insult to the injury of literally wasting dozens of hours across hundreds of thousands of players. That is the other way of looking at it.
This is par for the course for what I can only imagine is the C Team. Remember when they accidentally did the Conquest --> Honor conversion a week early without raising the Honor cap, resulting in thousands of Conquest points being turned into relatively useless gold? Or when MMR values were so FUBAR that they made the 2200 weapons require level 86, then delayed returning them to normal for months because not very many people were clearing heroic T11 content?
It is hard finding a stickie thread that is not also an apology for some massive screw-up.