1) Switch to Autosteel and remove all manual steel areas.
2) Switch to the new handling of OWW.
3) Remove empty space around the level.
4) Adapt falls of height 61-63pixels to make them deadly again (whenever I spot them).
5) Remove Fake objects and change invisible objects to visible ones.
6) Change all non-water objects to No-Overwrite, except if there is a strong reason not to do this.
7) Change time limits slightly, if the level is impossible otherwise (e.g. some of tseug's levels).
Anything missing on this list or changes that should not be made?
1. This is generally good, however there are some levels where this doesn't work as well. In such levels, often simply using "Simple Autosteel" can fix this. Two examples that are relevant from Original Lemmings - Steel Works, Autosteel fails because of all the non-steel terrain overlaid over the steel for decorative effect. For this one, Simple Autosteel fixes it up perfectly. Every Lemming For Himself isn't so simple, as there are some parts where normal terrain overlap *should* erase the steel area, and other parts where it's similar decorative stuff to Steel Works. In these cases, using steel areas at least to some extent is the only viable option. This is the reason for both the Simple Autosteel option (perhaps it needs a better name, though?) and the retaining of steel areas in both normal and negative form. For levels where the steel areas are not needed, rather than manually removing them all (although you might decide you want to do that), the "Ignore Level Steel" option comes in useful - it treats the steel areas as if they're not there, without actually deleting them (it doesn't affect autosteel; that will still be there if the level has autosteel enabled).
2. In most cases, they won't need to be touched. Some levels that have one-way walls on rough terrain may benefit from some touching up, as will those where the one-way wall slightly spills over onto terrain it shouldn't (but couldn't be avoided due to how L1 / Lemmix handled one-way walls). Just to be clear - using the one-way wall objects is still the recommended method; the option to have "steel" areas that place one way walls is more or less just there for Cheapo level import reasons, and has some significant disadvantages (most notably, that one way walls placed this way don't animate).
3. Yes, this is quite standard for NeoLemmix. In most cases, excessive decoration that takes up the empty space should also be removed.
4. NeoLemmix's fall distance is very similar to (if not the same as) that of CustLemm, so if most levels were designed with that in mind, this should not be a frequent occurance. Still, keep an eye out for it.
5. There are some cases where the objects being fake or invisible is the whole point of the level ("Check Your Hints" from Holiday Lemmings and "The Phantom Exit" from MazuLems both come to mind here). If it doesn't appear to be the whole point of the level, then remove them or make them non-fake / visible depending on the situation (eg. if they're fake as a side effect of the Z>16 = fake mechanic, and probably weren't intended to be, the right change would be to un-fake them).
6. Agreed.
7. I would think there aren't many cases where the time limit would make the level impossible, unless they were meant to rely on the pause-for-time glitch (in which case you can just add a few seconds). I would also suggest, on levels where time is not intended as part of the challenge (eg. relatively short levels with rather large time limits), just remove the time limits altogether.
I'd also point out that the general style in terms of skills with a zero quantity is to leave them there in packs that only use the traditional 8 skills, which all of these would be. It's only in packs that sometimes use the NeoLemmix skills that actually removing them from the skillset tends to be done. This isn't so much an official style recommendation, but it's how most packs have turned out in practice (though there are exceptions; eg. MobiLems II seems to work on a per-level basis - if the level doesn't use any new skills, all 8 old ones are on the skillbar, but if it does, then the skillbar may have empty slots).