If the mod is enacted it must be tested and documented thus maintaining the airworthiness trail.
That is precisely what the regulations say. And precisely what the people I talk about ruled need not happen. Especially on Service Engineered Mods.
The mandated Def Stan, subsuming the procedural specification detailing how to do this work, was declared obsolete 2 years ago, without replacement. It had not been implemented since 1992 and I challenge anyone in MoD to produce a copy! (Still got mine).