VERY DANGEROUS if done wrong. BACKUP before trying.
NOT FULLY TESTED
Only works for when a custom column is used
My example will use the custom column
#notes
1) Isolate books that have notes, Search
#notes:true (refine search as desired)
1a) Sanity check the results as to being ONLY the desired ones
2)Select All (results or those you wish to affect)
3)Tap 'E' (Bulk metadata edit), Switch to the S&R tab
4)
REGEX mode, Field is
#notes, Pattern to search for
(.+) <regex for everything, no replace value