So, last night I lost a couple hours to troubleshooting this.
I loaded a HF3Pro project into HF2017, and noticed that a bunch of my Missile Smoke effects stopped working. They were done in 2D comps, with the effect parented to a tracked point. The settings hadn't changed at all, but the emitters simply stopped, well, emitting. Changing the emitter settings didn't fix the issue.
Deleting and re-adding the effect, then re-parenting it to the point didn't always put the effect back onto the point, even after zeroing out the position attribute.
Even worse(?), it seems like if there are any 3D elements in the scene, the particle effect defaults to 3D space or something, as it becomes impossible to parent to the 2D tracked point; since the origin of the emitter is now in 3D space, it will be off-kilter due to the Z coordinate (I'm assuming). Parenting the point and zeroing out the position simply returns the emitter to 0, 0, 0 in world space rather than allowing it to track onto the selected point. I noticed a similar issue with the Sparks effect.
This is a royal pain in the neck, and I've had to go through my entire project and redo all of the smoke effects. Are any of these observations bugs, and if not, are there workarounds?