So I think I’ve figured out how to use it.
When selecting the Run Behavior Tree task in my behavior tree, I was able to create the Gameplay Tag “Test Tag”, which can also be done in the Project Settings (I accidentally created a sub-tag called “Test” that I wasn’t able to delete, but it wasn’t important).
Then in the AI Controller for my character, I used the Set Dynamic Subtree node and selected the same tag as is defined in the behavior tree. This managed to change the behavior asset after runtime, and I was even able to make it swap back-and-forth between two trees.
I did try this in a different blueprint other than the Controller, but it wouldn’t work for me; only seems to work in the Controller blueprint.
Hopefully this works for everyone 