Update: Apparently this is by design and can't be avoided due to the changes between the processing versions. Wow, Lightroom 4 is really off to a less than stellar start if you ask me.
Doesn't anybody at Adobe work with custom tone curves? That's hard to believe. I have just found a second issue with them.
Adobe released Lightroom 4 and they pretty much messed up the migration of tone curves when converting the catalog from Lightroom 3 to Lightroom 4. You can read all about it here, the story is still ongoing as of writing this.
I use custom tone curves a lot, so I filed the original bug report right after finding out about this issue.
This is why I'm now part of a group of people alpha testing a fix that should recover the lost tone curves after an upgrade and that will hopefully make it into the full version of Lightroom and into an update for those who already upgraded.
As far as I can tell, Adobe hasn't issued a warning about this to their existing user base and we can only hope that power users with tens of thousands of pictures (and potentially with as many tone curve adjustments) won't get too many nasty surprises due to the bug.
Here comes issue #2:
During testing of the alpha script, I noticed something else, that I find quite disconcerting: I know changing to the new process will change the appearance of pictures, which is why Adobe suggests an A/B preview, but when I had the tone curve open when switching a picture from process version 2010 to 2012, I noticed this:
The curve does keep its overall shape, but the quite elegant few points of the curve get replaced by a ton of individual points.
WHAT .. ON .. EARTH .. IS .. THIS?!
Doing a quick change to the mid tones, or to how the shadows are rendered is a simple fix with the original curve. The replacement curve is 100% useless for that.
The only way to make the curve usable again is to start over and re-create it from scratch.
If this is by design, then it means that those of us who use custom tone curves extensively (I'm one of them) won't be able to benefit from the 2012 process for any of their existing images unless they are ready to start from scratch on them. In that case I'd really like a word with the person who made that decision.