Word 2016/office 365 on Windows 10 Pro, all up to date (I think)

Document c. 80,000 words

This document has many (>200) double quotes, e.g. "text", which my editor has asked me to change to single quotes, e.g. 'text' using Review, All Markup.

I proceeded with Find (space)" and this located the first one to change by typing ' with " crossed out (correctly). I then went to the end of the quote and did the same there, then Find again. After a number of such corrections, always apparently during the second change in a quote, Word would hang completely and the rest of the text starts to be slowly markup-deleted, letter-by-letter. By 'hang', I mean absolutely, no way to do anything, including leaving the programme. I close down the Task View screen with Word to stop it or Ctrl-Alt-Del.

I ran the MS short 'cure-problems' app: no difference