Well well, it's the legendary bulletride, who has returned after a long absence. It's great to have you back, and a pleasure to finally meet you!
I've heard plenty of wonderful things about you.
Alternatively, with an older version of the NL player, version 12.9.4 or older I believe, you can put your .lvl files in a subfolder in the "levels" folder of your NL directory, load up NL, find the subfolder in the "levels list," load it by pressing "ok," escape out to the main menu, then press f8, which will cleanse all the levels you have in the subfolder. The .nxlv files will then be found in a "cleanse" folder, which you should be able to load in the newest editor, but if they don't open, then you can do the same thing in the 12.12 NL player, but with the .nxlv files, except there will be a "cleanse all" button in the "level select" menu like Armani said, making it easier so you don't have to memorize the hotkey for "cleanse." This should allow you to open your levels, like you reported
Only downside here, of course, is separating all the .lvl files from the .nxlv ones afterwards, but then again it's essentially the same as loading the .lvl files individually in the editor and saving them as .nxlv files.
This, btw, was how I converted your Lemmings Stampede pack to New Formats NL
I'm going to presume that you have indeed seen the finished product, as well as the video series that I put together for it. All info on this you can find here:
https://www.lemmingsforums.net/index.php?topic=5709.msg93022#msg93022Also, I posted my replays and feedback for your pack for Old Formats NL here:
https://www.lemmingsforums.net/index.php?topic=3200.msg93090#msg93090 . Alternatively, I posted all my solutions to your levels on my Youtube
Along with this, would you happen to have the old Wicked 10 level
Bobbing for Lemmings by any chance? I would love to give it a try sometime
If not, no worries.