I've been using the code supplied by NXP with the LPCXpresso, so I can only guess why one works while the other does not.
The datasheet states:
"In order for the update to take effect" after switching PLL input source "first write a zero to the SYSPLLUEN register and then write a one to SYSPLLUEN."
So it is not clear why it is necessary to write a 1, and only then a 0 and a 1 again, like is implemented in the working code. Also, it is not explicit that one would have to wait until it reads 1 from the register. It looks like the new file exactly follows the datasheet, leaving out those extras that are not obvious.
I'm also new to this and maybe those 'extras' are common practices while dealing with this kind of processes.