Ten ideas on why Marion could have changed her name, in no particular order.
1. Safety while travelling alone. (The woman who used Marion's stolen credit card may have committed other crimes in her name that could cause her trouble while overseas).
2. Mystery man asked her to do it as a favour to inherit something/make happy elderly/sick relative (false story)
3. A name like Florabella would be a talking point while overseas, would engage people in conversation, "oh, what an unsual name", get's the ball rolling.
4. Advised by someone in a position of authority/power to do it.
5. Would feel safer travelling alone overseas with a name that looked like it was from there, name sounds European.
6. Didn't want anyone to know about her previous marriages, so a bit of a clean slate, and why not go with an interesting name.
7. Had a serious bet with someone, lost, and had to honour it by changing the name. (bet with villain in the case)
8. Didn't want to be associated with her past life anymore, wanted to start anew (maybe had bit of a breakdown)
9. Was threatened by someone who said they had a contract out on her - hey, a bit outrageous, but I've got to get to 10.
10. Was asked to change it to deliver something undercover for mystery man overseas, government secret stuff - (well, could be a story that she was told by mystery man, who knows)
It would seem that the name change only appeared on the passport, which is interesting, doesn't appear to have come up anywhere else, which makes you think it was just for travel safety/advised by someone else, again this mystery person. I don't think Marion thought of this by herself, the answer to everything is probably with who advised her. Ah well, just a few thoughts. Cheers