I have the impression that in dictionary of 80's 帧 was still labeled as "zhèng" while in the 2000's it's been changed to "zhēn". There are also similar changes such as "qián" -> "xún" for 荨 in 荨麻疹, etc.
Question: When did the change for 帧 and 荨 happen specifically, and where to look up for an official document for such changes in general.
There's a short Sina blog post that contains some explanations.
There was a《普通话异读词审音表》that was released in 1985. One of the words on the list was 帧:
帧。“装帧”指书画等物的装潢设计。《集韵·映韵》：“帧，张画绘也。”猪孟切，字书据此注为zhèn。声旁是贞，通行的读音是zhēn。《初稿》未审，《定稿》读为zhēn，与贞、侦读音一致。
On the same document you can also find 荨:
荨。《初稿》审订“荨麻”一词，定“荨”读qián，不取xún音。“荨麻”是一种植物，人如果被它刺伤，皮肤就刺痒起疹，成为“荨麻疹”。医生和病人把“荨麻疹”读为xúnmázhěn，与“荨麻”的读音不一致。《定稿》把“荨”字的读音分为文读和白读：文读读qián，如荨麻；白读读xún，如荨麻疹。
Wikisource has 普通话异读词审音表（修订稿）[2016] posted in full plain-text. Along with the original 1985 version.
