DV video, as found on MiniDV video cassettes, is already a lossy compression, you can't re-compress DV files without introducing another generation. Period. You may decompress the DV material, and re-compress it with a more up-to-date compression scheme, but that, again, is another generation in terms of compression and quality loss.
What you can do, though, is decompress it once, and re-compress it in a lossless way with a more recent lossless codec, hopefully more efficient than the lossy DV compression scheme. This way you decompress once and then never again. Here on the blog is a comparison of lossless video compression codecs which might be helpful.
(wip note: this could use some testing)
However, you can also compress the whole MiniDV data stream once you have it offloaded from the tape, for example as a .7z file, just like any other binary data.
(wip note: this idea could use some testing as well)
Many people would argue, that keeping your DV video data on tape is a very solid long-term archiving solution. While
others would argue having the data as a file on magnetic media is less prone to rot or fungus, at least when compared to
tapes not properly stored.
A win of having the video as a file vs. the video as a stream on tape is, as a file, you have proper random access to the data,
without shuttling the tape.
As you probably already noticed, this post is merely a collection of thoughts and ideas, and pretty much a work in progress.