A really quick Blender addon to rename "hidden" packed textures. It can rename materials after the object they're on and textures after the materials they are attached to.
Download rename_materials_textures.pyto your computer and install it in Blender via Edit > Preferences > Addons > Install.... You're ready to go!
The renaming operation can be found at File > External Data > Rename Materials/Textures.
From here, you can choose to rename either textures or materials or both. Note that the operator only handles selected objects!