Funny coincidence, I was just playing this level.
If all you're doing is making a fix, and there's no possibility of wanting or needing to return to the previous version, I think you should keep the same filename so it saves over the old one. I've already accumulated several versions of this and Stick-up, and decided to declutter my wip folder by keeping only the "main" ones. If you should happen to change your mind, you can always re-download old versions from this thread.
Time to reveal your solution, perhaps? I feel I've already spent enough time on these two levels, and I want to get back on with my own now. Talking of which, here are some more. "Untitled Columns Level" (yeah, I know...) dates back to my ResEdit levels (before I even knew about Cheapo and other level-designing tools, I built levels by editing the standard ones). This one, originally called "On the Lemdon Underground", had the layout of Fun 26, with 1 floater, 6 builders, 2 bashers, 2 miners -- and the uninteresting solution of making the first lemming mine immediately on landing, which at least on the Mac version allows him to get deep enough to build to turn him and the others round -- yes, this was when I still thought pinpoint precision was Teh Awesome, though as I've said, I still like it if not overused -- while one floats, bashes, builds up to the level of the exit and bashes to reach it; then mine to release the crowd. (Spot the backroute!) For my Cheapo sets, I rebuilt this level, with a different title, with a slightly more complex layout (though the similarity to Fun 26 is still fairly apparent) and a more interesting solution -- in fact, when I looked back at this level after not having seen it for years, it did take me a while to spot the solution, so I suppose it's an okay puzzle. As for the title, I can't use a Lemmings-based pun in Lix, and I don't like its second title as it's been used before and gives too much away; so it's untitled for now.
The harder version is mostly bomber timing (and also dates back to my ResEdit levels). Sorry, but I do like playing these. No-one answered my question about whether making a bomber timing sequel to "Charge of the Lix Brigade" would be okay, so I made one anyway; I'm not going to force it down your throats, but if you want to include it, here it is.