Just to clarify: Pit Lems was made entirely prior to New Formats. I just initially thought it was not worth converting, that the levels weren't good enough. My perception regarding this has changed after I noticed that most of the custom content for New Formats is quite challenging, so it would be helpful to have more intermediate-level content for NeoLemmix beginners.
The other thing is that there are still many unconverted packs by other users, and occasionally, someone more active in the maintenance of packs, like IchoTolot, picks up converting one of these. I'm referring to things like SubLems, Lemmings Stampede etc. here, not my own packs.
So that's another purpose for which the NXP converter would be useful: Slowly transferring content by users who are no longer active, but who did contribute acclaimed packs back in the day (such as Lemmings Stampede), to New Formats - without the person picking up that task becoming overwhelmed by it.
That said, my state of knowledge was indeed that conversion of old content would be possible up to 12.10, meaning both via the level editor
and the pack conversion tool. If it only refers to the level editor, then of course I still have the option to convert the levels by hand, one by one, by opening them in the editor. I will have to do that anyway with the levels I plan to add to the New-Formats version of Pit Lems - it simply would have saved a lot of time to have the original batch of 100 levels converted by the pack conversion tool.
I don't think there's a way to make the converter "skip" levels that produce errors, so that I could simply convert those selectively by hand later?Of course, I could use the old pack toolkit to make a "transitional" Old-Formats version of Pit Lems without the level that causes problems, then simply convert that, and add in the missing level via the editor (which can open it without any issues, as explained in the starting post).
I'll just have to see whether this is an issue that affects every level containing default pickup skills. As said before, I knew that custom pickup skills would cause problems, but I didn't expect the default ones to act differently (because they didn't do so either when I converted Lemmings World Tour).
Speaking of LWT: All the graphic sets are already converted, so that's not the cause of the issue.
So I'll probably just have to remove all levels with pickup skills from the NXP, just to be on the safe side - then convert the NXP, then add in the couple levels that do contain pickup skills manually via the editor. 
