Passengers transferred from the Titanic before sailing

Family legend has it that Grandmother Miss May Pattison was due to sail on the Titanica but sailed on a different ship from Liverpool on 10 April 1912 on a White Star Line ship. Is it possible to trace a list of passengers that changed their travel arrangements ?
Dear Kevin - As far as I know there is no list of passengers who changed their travel arrangements.

However there are plenty of genealogical resources out there to aid you in finding out more about your grandmother's journey.

Was your grandmother travelling to New York? I did a very quick search on the Ellis island website and couldn't find any records for "May Pattison" or "Mary Pattison" entering the USA in 1912 but sometimes these searches take time.
Kevin, the Ellis Island database has two New York arrivals listed under the name May Pattison. One was a 14-year old American citizen heading for Rochester, NY in 1896 on the 'State of California' from Glasgow. The other, listed as May Cunningham Pattison, was a 32-year old Australian citizen en route back to Oz in 1918 on the 'Balmoral Castle' from Liverpool.

Within 5 years either side of 1912 the only arrival of a Mary Pattison was on the 'Baltic' (White Star Line) from Liverpool in August 1914. She was British, aged 27 and married. Born in Newcastle but had resided in Brooklyn, New York.

Any suspects there?
I think they've transcribed the age wrong on that last one. Looking at the handwritten manifest sheet I'd say it was 37, not 27. Mary Pattison was traveling with Claude Pattison (same age), a salesman. Probably her husband.
Thanks to Derek & Bob for your help,I made a mistake with my question and know when Grandmother sailed, where from and where to, and which ship. (10.4.1912 Liverpool, RMS Cymric bound for Portland). All of which confirmed large part of family legend which is why I am hoping that there might be a list of cancelled passengers or similar especially as she sailed on a White Star Line ship.
Glad you found out which ship she was on.

Interesting note about the Cymric: She was sunk on May 8, 1916 by U-20 (same boat which sunk the Lusitania) - one year and one day after the Lusitania's sinking.
Kevin, if any such list ever existed, I'm afraid that by now, it's probably long gone. A century's worth of time is not kind to records. If you ever uncover any such, it would be quite the remarkable find!