Bill,,
What happens for experimentals undergoing a major change (certified before the new wording) depends on what your local FSDO put in your specific operating limitations.  There appears to be a lot of local discretion here.  A few weeks ago, I had a half hour discussion with my FSDO on what kind of change would require recertification, as described in my insurance policy.  Their reply was that there is none that would require 'recertification'.  My operating limitations only require me to notify the FSDO of any major change per 21.93 and wait for a reply.  In FAR 21.93 a major change is defined as <<A ``minor change'' is one that has no appreciable effect on the weight, balance, structural strength, reliability, operational characteristics, or other characteristics affecting the airworthiness of the product. All other changes are ``major changes'' >>
I used to believe the definition for 'major alteration' as defined in 43 App. A was applicable, however in the applicability section (43.1) experimentals are specifically excluded.  I also spoke with my insurance company, Avemco, who told me that as long as the FSDO was happy, they were happy.
So the bottom line is, know what is in your operating limitations and talk to your local FSDO.  They can all be different.
Chris Zavatson
N91CZ