The 747-400X uses the same navdata files as the FS9 version. Navigraph's installer is already updated to work with the 400X,
(there's a radio button to choose between FS9 and FSX) and sources like
Terry Yingling's Planepath files or Dan Downs' custom files available at AVSIM work just fine too.
One for sure way to do it is just copy over the files from your
FS9 SIDSTAR folder after performing whatever update... Be aware though
that the location of the AIRAC has changed for the FSX version - the
files are now in:
<FSX root folder>\PMDG\NavData
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