To do this easily, and there may be easier ways than my way - it does only take a couple of minutes tops for all the samples in one song:
Download the latest version of OpenMPT (free, and at the time of this post the latest release was Aug. 20 this year):
https://openmpt.org/download
Open ancient/noncompliant module.
Go to the Sample tab, and for each one select Resample in the Edit toolbar:
https://wiki.openmpt.org/Manual:_Samples#Edit_Toolbar
The Resample dialog box appears:
https://wiki.openmpt.org/Manual:_Resample
Enter 44100hz (if you’re trying to make stuff Polyend friendly!). Choose a resampling filter and click OK - bearing in mind, the filter algorithm has a considerable effect on the “quality” of the result - with quality being subjective if we’re talking about oldschool samples. The default “r8brain” setting is pretty good for clean-sounding material but if you want an especially gritty DOS/Amiga 8-bit digital feel… try linear, cubic or none at all.*
One you have done this for all samples (thankfully it retains the 44100 setting you typed in the first time), click the dropdown next to the floppy disk icon (still within in the Sample page) and select Save All.
Now that you have all of your samples exported somewhere and at 44.1kz, time to up them all to 16 bit. Lots of free tools will do this. I chose Freac (supported on Windows/Mac/Linux), here:
https://www.freac.org/
In Freac… among the bottom options, change Selected Encoder to “Windows Wave File Output” and press the gear icon next to it to configure - set it to “signed 16-bit PCM”. Drop in your exported samples, and click the play icon at the top to convert.
If you don’t like the file naming, in the Configure menu at the top of Freac - under Encoders - change the stuff under Output Filenames.
Boom, all that hideous mess is now ready to rock in Polyend Tracker, Octatrack or whatever. And after resampling everything to 44100 in OpenMPT, modules seem to import better even if they remain 8-bit. I still had to change the sample tuning to zero on many samples in Polyend Tracker but no big deal, I have yet to try reimporting the 16-bit versions back into the module, might fix it since it’s the manual’s requirement. Tempo and sample volumes were still all over the place but again no problem; the way the Polyend handles gain seems very different from software trackers I use so I adjusted for that plus their nice FX sends.
It probably takes longer to read my walkthrough than actually do this conversion 
Worth the details if it helps others.
*For more on resampling filters, read this from the FAQ:
" OpenMPT must resample all samples to a common mix sample rate (which is typically 44.1 or 48 kHz). This resampling step can be done using filters (interpolation) of varying quality. A perfect filter would not add any frequencies to the output signal that were not in the input signal. But such a perfect filter does not exist, and OpenMPT instead offers several filters of varying quality and speed (higher quality filters stress the CPU more than lower quality filters).
If modules were only using high-quality samples that already contain enough high-frequency content, i.e. samples that are being played close at the mix sample rate, you could always just use OpenMPT’s highest-quality resampling filter and everything would sound bright and nice. However, many modules rather use samples that use a much lower sample rate, which means that these samples theoretically do not contain any high-frequency material. But when using a low-quality resampling filter (e.g. linear resampling, as it was used in many trackers in the 1990s), the cheap algorithm cannot prevent ghost frequencies (so-called aliasing) from occurring, so these low-quality samples end up sounding much brighter than they should in theory — this is a wanted effect in many legacy modules. On the other hand, using a high-quality resampling filter will suppress these aliasing frequencies better and the module will in return sound muffled.
This is why older modules and chiptunes typically sound better with “worse” resampling filters, while a high-quality module with good samples will sound better with OpenMPT’s best resampling filters. For Amiga module formats, you may enable the Amiga Resampler for accurate reproduction of the Amiga’s sound characteristics."
