Unless the law has changed in recent years (and I don't believe it has), you are required to update best mode in a CIP. Jeez, it's been a long time since I read that case, but I believe the requirement applies to all matter disclosed in the CIP, whether new or old.
Perhaps it was
Applied Materials Inc. v. Advanced Semiconductor Materials America, Inc., 98 F.3d 1563, 1579 (Fed. Cir. 1996).
At least that's what turned up when I tried to answer the question myself.