I can see both sides here. For a start, I agree with Dullstar that there are some decent levels that didn't make it into Redux -- either because the levels I like aren't popular with the crowd, or ones that just fell through the cracks of the selection process. At times I've considered reopening the question of whether to add more levels, but every time I've decided it isn't worth doing so. For starters, I believe the current selection is really strong and I certainly wouldn't like other levels to be included instead of any of them. Also, 160 levels (and 32 per rank) is a decent size for a pack and I think we shouldn't overload it. Since we're close to a final update, I will at least ask how other people feel about this, just in case, but I think it's unlikely we'll end up making any changes at this stage.
So, where does that leave the official level packs themselves? I definitely want them to continue to exist in some form -- for completeness, for the sake of challenges, and as a resource for making custom levels. I'm not that bothered about the concept of playing through the original levels in NL as a pack (other than maybe doing it as a speedrun some time) but I think we should maintain the levels for those who want to play them, since this will be a relatively low-maintainance effort (no need to fix backroutes, for example).
As for what changes to make: I think the original, untouched levels should be available for download as a resource, but the main pack should have certain changes to make it a NeoLemmix version. I would be happy to cooperate with WillLem in getting this done, even though we will disagree on exactly what changes to make -- but when that happens, we can always ask for community feedback.
Honestly, I think the existing conversions are mostly fine as they are, and if I were in charge, the only change I would make to them would be to restore the decorative terrain on a small number of levels where I think too much was removed. I don't mind WillLem redoing the conversions from scratch, but it does seem a little pointless to redo all the work that's already been done.
The one place I absolutely draw the line: when it comes to moss on steel, we have to respect that in NL this doesn't work any more, and it must be removed. I'm not happy about the decision to deprecate simple autosteel, but in the end, I understand why it was done; I respect that it's a settled decision now; and we should be consequential and remove the moss from old levels (both the official ones and any other old packs that are still being actively maintained).