How to batch rename some words in XMP tags?

[Eyael]

Hi,
I'm using XnviewMP and I have ticked the option to export categories to xmp tags within the images. Works fine. Now I've just had to rename some categories adding a letter at the beginning so that they are sorted on top of the list for easier access. For the xmp tags to remain correct I would need to edit them which would take forever. I have downloaded exiftool and the GUI that goes with it hoping it would be the right tool but I just can't figure out how to use it and whether what I'm trying to achieve is possible.

Now let me give you a more concrete example of the task I have to do. I have a folder named "Images" (very original!) which contains several subfolders with pix, no pix at root and a folder named "Kits" which contains about 350 subfolders with pix (still no pix at root). Some images contained in all subfolders have the word "Embellissements" in the hierarchicalSubject tag (XMP) and I would need to rename this part to "@Embellissements" just like you woud do with a file renaming tool except I would rename the contents : find all the files containing this word in XMP tags and rename this word to another. Is it possible with ExifTool ? And if so would someone be kind enough to tell me what to enter in the command line? Thanks very much for any suggestion.