In recently merging my Ancestry.com gedcom with my Legacy file, I encountered an issue, where many people had Y inserted into the Death Notes as this was on my Ancestry.com tree. (whole other story as to why that was).
Legacy will not AutoMerge any two profiles when one has the "Y" in the Death Notes and the other does not. With a huge file like mine, this can be a major issue.
So, to get rid of the "Y" I used "Search and Replace" in the Death Notes field for "Y" with whole word and match case checked. I left the Replace With blank, and this got rid of almost 28,000 of those pesky Death Notes.