Taken from the usa forums, eeeek so me hoping to finish my heroic mog went out the window : /
Seems like we cant catalyst items now, am like 200% sure in the past we could still get mog items 
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Item upgrades are disabled when the season ends. It doesn’t makes any sense for me either, but most likely related to currency reset / removal. Anyway, the same way it worked in previous seasons.
Regarding the catalyst: it was possible to transform items from previous seasons for free (without any charges required). I guess you just need to wait for season 2 to start (unless Blizzard introduced some additional changes 
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Kaivax
1778 posts
Hello! We’d like to acknowledge the unusual situation here, where Season 1 gear upgrades are no longer available.
You’ll find that world content in in the new zone gives Season 2 gear, which can be upgraded. There’s some awkwardness to the one week transition period, but as of the patch going live, we need the world to be in a “Season 2” state in terms of currencies so that the Undermine zone content can function as expected. At the same time, we’re keeping Season 1 instanced content active just to make sure players have a few more days to finish some of the achievements that are still available.
We don’t currently have a way, technically, to keep Season 1 gear upgradeable after moving into a Season 2 currency ecosystem, which we have now done.
We understand that this can be confusing and frustrating, and we’ll continue to discuss further improvements to season transitions in the future.
From the usa forums, so It looks like you cant even mog s1 items?
Aye, it was like that in DF, I do hope they bring that part back in S2, only time’ll tell 
What is worse is it activated last night before we even got the patch. Did a dungeon with a guildie and none of us got crests.
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All gear should be upgradanle no matter the patch, especially because even if you full upgrade an Heroic item from patch 1 it will be worse than a patch 2 one because of itlv inflation.
I just catalysed the one remaining piece I needed for tmog set so new charges are still happening.
Edit: So what Blizz have done is allow players to keep going for S1 achieves even though S1 is now finished. In hindsight, they should have just made this reset the official end of all S1 stuff and made it clear that’s what was happening.