It is confirmed. It's on page 36 of the manual. How long does it take? Does it matter if it takes 30 seconds or 3 minutes? It would ruin a performance. Owning more than one expansion would probably just be of interest to a home player.

It would be so frustrating to own more than one expansion, and to know during a performance that you cannot take advantage of those voices and those styles because it's not loaded on your measly 64mb Expansion slot, and you'd have to turn your keyboard off and on (the keyboard restarts during the process) in order to play them.

Here again are the two quotes from page 36:

This instrument allows you to
install only one Expansion Pack.
Installing an Expansion Pack will
erase all the previously existing
Expansion Pack data in the
instrument. Make sure to keep a
copy of the Expansion Pack data
in a USB flash memory for future
use.

The instrument will be restarted
when installation is completed.
Save all the data currently being
edited beforehand, otherwise
the data will be lost.