Many will fit the standard cutout. Take down the existing one and measure.
See if the Panasonic is the same size..they will vary

The instructions on the fans all talk about new installs and usually
seem to require that you either open up the ceiling or have access
from above.

Do bathroom fans typically use a "standard" size that would allow you
to swap in a new (quieter) version or does one tend to have to rip
open the ceiling and install a full new unit?