This is something I’ve thought a lot about. When making beat-oriented music, a firm sense of structure – telling the listener where they are in the song – is really important. I think of it like text: each 4 (or 16) measures is like a sentence, which needs punctuation at the end; and the larger transitions you mention (between parts of a song or time signatures) are like new paragraphs, which require punctuation and some sort of space or transition in between to orient the reader.
I grew up listening to IDM in the 90s, so my ideas about transitions are firmly rooted in that genre – YMMV, of course. What I like to do to create a strong sense of transition is to start mangling the drums a measure or two before the change, with the intensity of the DSP stuff rising as the transition approaches (but, crucially, leaving open the possibility of flowing into and through the transition, maybe ending half a measure or a full measure into the transition if that feels right). There’s also the classic Aphex-ian “slamming into a brick wall” method where the drums abruptly go into chaos right before the transition. Standard DSP methods for doing these things include comb filtering, glitching effects, delays, spectral effects, etc.
This is probably easiest in computer-land, where building complex layers of DSP stuff is easy (Sugar Bytes Turnado, Native Instruments Molekular, and Unfiltered Audio Byome, Triad, and Spec Ops are one-stop shops for this kind of thing) but you can certainly get there in modular as well. Of course filter sweeps are available in modular, and lots of modules give you comb filtering and delay. I’ve also been loving the Qu-Bit Data Bender, which gives me all of the classic IDM glitching I want in modular. Really, it’s all about fucking up your drums as hard as you can right before the transition, so that the listener feels oh yeah, things are changing.
Here are 3 concrete examples: AFX’s Box Energy Remix, Menelec, and Vatstep DSP. Sometimes I think that 40% of my music making time is spent simply trying to achieve the transitional effects AFX gets in that one song. (Note how he inserts a chunk of heavily reverbed dialogue as a transition to the last part of the song… chef’s kiss)
n.b. In modern EDM parlance I guess this all falls under the heading of “risers,” but I don’t like thinking about things that way because risers are usually followed by drops and I hate drops.