Good question! I’m not entirely clear on exactly what type of changes you are asking about, but you are right that trenches are dynamic, meaning they change over time.

A geologic trench is the location where an ocean tectonic plate descends beneath a continent or another ocean plate. Ocean plates descend into a trench when they become denser than the mantle rocks that are underneath the plates. This causes them to sink. One way that trenches change is that they move. Sometimes they “retreat” steadily and sometimes they “jump”. I have included an illustration that shows both of these processes. trench migration Trenches “retreat” when the ocean plate that is sinking “rolls back” like a dog’s tongue hanging out of its mouth on a hot day. This causes the trench and the upper plate to be “pulled” toward the ocean (stage 2 of the trench retreat illustration). Trenches can “jump” when a buoyant piece of crust riding on the sinking plate moves into the trench (stage 1 of the trench jump illustration). Because it is buoyant (less dense than the mantle that it is floating on), it can’t sink and clogs the trench. This causes the ocean plate to break and start sinking behind the buoyant crust (stage 2 of the trench jump illustration). These are two examples of how geologic trenches change.

Ocean trenches are subduction zones, zones where one ocean plate is diving beneath the other. This creates a furrow where the diving plate goes in, and thus furrow is a trench.

The other is trenches that geologists dig in order to look at layers of sediment. Changes here happen due to different layers being laid down for any number of reasons (typically storms, floods, earthquakes, or other events).

I am unsure of which changes you mean exactly, but I can talk about a few changes that happen to trenches. Trenches form where one plate is subducting underneath another one. The trench fills up with sediments which are either scraped off from the subducting plate, sediments coming off from the over-riding plate, or from regular ocean sedimentation. So the trench may change as different sediments are deposited into it. Also, tectonic plates are always moving. Each one moves in a direction at a different speed (we can track this today using GPS!). The trench location may move and change depending on the plate movements.