That name may very well change.
It may change more than once.
There may even be name changes for different markets (see Coz Greenop's story!) - US/UK distributors often use different titles.
But there is ALWAYS a working title by which to refer to the project.
There are many famous examples of movies that changed titles at any point in the pre-production, production, post-production or distribution phases (a distributor is staking money on successfully marketing a film, if they think the title is harming the branding they will change it):
- Halloween was conceived of as The Babysitter Murders - the producer had the idea that everyone experiences babysitting, and managed to get John Carpenter to come up with an actual script for this very basic idea
- Scream was initially Scary Movie - a name then used for the postmodern satire on Scream
- Huge hit and franchise starter Alien may well have flopped if its working title had been retained: Star Beast!