Never mind, I figured it out. It's caching right now, so I'll try to recount it from memory:
-Create a new emitter, give it a new name.
- Go to Emitter Shape, choose instance, and drop the older emitter into the slot that becomes available.
- Create a new cache object, place the old Emitter into the appropriate "Exclude" tab
- Create a new cache tag on the new emitter, and reference the new cache object into the appropriate slot
- Adjust your new cache object as needed, and cache away.
It's still a bit slow to re-cache; ~ 7 seconds per frame for a few million particles., even on an SSD. Then in Houdini I have to reconvert the bgeo sequence to bgeo.sc files, which are half the size. I'm running that process at the same time, and it's taking less than 1 second per frame.