I do this occasionally while doing a solo gig, drop everything down to C# or C. I've gone lower, and the trick is having a set of strings that tunes to a fairly high tension. I've had great success with Pirastro Tonica strings; they don't break the bank, have a nice tone (perhaps not the most 'trad', but quite nice), and since they tune in at a fairly high tension, you can drop them down in pitch a good deal and they don't become 'rubber bands'. Thomastic Dominant strings work well for this too.
Word to the wise, however, is that higher tension strings (in my experience) will 'stretch' and 'settle' more than lower tension strings, so if you're installing a set of these, it's best to do so (if possible) about 2-3 days before you need to play, and play them in or at the least tune them regularly so that everything stabilizes. If you don't and you're trying to use scordatura in a gig, you may have some issues getting everything happy.
Transposition works well if you can do it, but the timbre of transposing on a GDAE violin is far different than playing on a re-tuned violin. Re-tuning can also make your doublestops sound a lot more pure since you'd have easy access to open strings that will fit with the tonic, subdominant, and dominant, (assuming you're playing a tune in D, G or em, which is pretty common in the ITM scene), and to my ear, the timbre of a down tuned fiddle blends much nicer with flat pipes or other instruments in a flat session. For non ITM music, it likely makes more sense to just transpose, but I'm assuming we're discussing the violin in the ITM setting

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