Combining Jeff's input with Paupa's - we build these planes and obtain and maintain an AW cert by following the rules. One of the rules in our experimental airworthiness certificate says that 14CFR 21.93 applies (in mine it is in para 14). So, if one were to add anything in hopes that it would alter a flight characteristic(s), putting the plane back into phase one and performing and documenting the required tests should put you back in exactly the same certification/legal position you were before the mod - wouldn't it? To me it boils down to: any and all changes made during construction were incorporated in the EXP AW certificate (phase 1, et al) and any changes made later maintain the same level of authorization if they are made IAW the rules.