I highly doubt that such a script would be all that useful.
…let’s create it anyway! 
I have modified one of my other scripts so that it now appends the old mnemonics after the new ones in lessons, reviews, and on the item pages. I did not want to spend much effort on this (it already took way longer than intended), so the script is not that well tested. In case anyone uses it and notices bugs, I can try to fix them.
