Here’s a video I did on the subject:
In it, I switch from ik to fk over the course of a few frames. Switching seamlessly in the space of 1 frame can be done when switching from IK to FK, but not the other way around, FK to IK, in 1 frame. I know the crew working on tube use a custom python script to switch from FK to IK in one single frame. If that is what you are really after, I think that feature is now in rigify. You’ll need a more recent build of blender to get that feature, then you’ll need to figure out the script and how to adapt it to your rig. I was going to look into how this is done and figure it out, but haven’t had time yet, so I don’t know how it’s done…
Randy