I could be wrong, but I think the more time-consuming part of certification is initial certification of the procedure - a company (eg a company that offers modi

I could be wrong, but I think the more time-consuming part of certification is initial certification of the procedure - a company (eg a company that offers modifications to aircraft owners) will gain certification for their ability to perform a specific procedure on a specific aircraft type. They can then do that procedure on customer aircraft. I believe the certification of the aircraft itself is less time-consuming; more of a confirmation that the approved-upon procedure was followed.