Harlequin's policy was to not disclose the change in title, at least until the 17 retitled Mills & Boon reprints when the original titles were noted on the copyright page.
Of course Harlequin was not alone in looking to boost sales with a new title. Here's an example where three paperback editions of a book have three titles, including the original. It's interesting to note that one edition is "abridged", one "revised" and, as usual with Harlequin, nothing is said about their edition.
There are Dead Men in Manhattan was first published in 1946 by New York's Mystery House, an imprint of Arcadia House.
Harlequin 115 - April 1951
Harlequin 115 back
Graphic 42 - 1951
Avon 772 - 1957