Of course the business reasons to do a project might change. It might become more interesting to do a project, or less... It all depends on the environment you're in:
* the strategy of the department or larger organization might change, e.g. it's no longer our core business; or new responsibilities make the need for the new system even more important.
* there is a change in legislation that forces us to do more; or makes the desired benefits less likely...
* the competition has launched a similar product or service, and is now claiming a part of the market you were hoping for.
It's inherent to a project that it involves change, so also the business rational might change. As long as there still is a business justification, it's fine. PRINCE2 does not state it should be fixed! But rather it should be reviewed regularly, and verified if it still makes sense to do/continue the project.