How do I edit an ASCII FBX as text ??

All too often, I get FBX files, some exported from Blender plug-ins, with missing texture calls or calls that are 'absolute' rather than 'relative'.

Not a problem if only a few but, if import to Poser spawns dozens of sub-rigs, each with a material that calls multiple texture maps, it can be a tad discouraging.

This absolute / relative issue often happens to OBJ+MTL exports, too, but I can sanitise those with Notepad (<50 kb) and Wordpad (>50 kb).
And they work...

FBX is a very different matter. For whatever reason, editing any ASCII FBX with either Notepad or Wordpad seems to create an invalid file.

eg I can do search/replace on the multiple texture calls routed deep, deeep into author's I: drive in a matter of seconds, but the saved file is 'broken'...

Any ideas to what is wrong ??