From what I understand of LO (I’m neither a guru nor a developer), no conversion is at stake because there is basically no difference between footnotes and endnotes. The note paragraph is tagged with a style name which is defined by Tools → Footnotes/Endnotes... settings. There you have two tabs for customizing two behaviours. Whether they dictate the set notes at the bottom of the page or at end of a document is a matter of setting a flag (clicking on a radio button). Both settings may result in the same layout despite the names.
Of course, if you use Endnote style for a foot note results in user confusion.
In your case, either you misused the style or started your document with one layout and later changed your mind, it does not matter.
What you really ask for is the ability to change globally the name of a style to avoid to hunt all occurrences.
Have you tried Edit → Find & Replace..., More options, clicking Look for paragraph styles? You select the offending paragraph style from the Find drop-down menu and the replacement paragraph style in the Replace drop down menu.
If this answers your question, please tick the check mark and close the question for community benefit. If you find it valuable, be kind enough to upvote it if you have enough karma.
