Switching between IK and FK rig

[AlueeN]

I was doing a walk cycle.First thing I was doing was using IK for animating legs.After I was done with the legs I switched to FK to animate the rest of the body.After I was done with that I switched back to IK but keyframes were gone.I'm confused how this whole thing with switching rigs works.I suppose that happened becuse I altered the legs animation using FK rig too.

[Zappy]

Keyframes are deleted, but animation retained, when attaching or detaching (most?) rigs.