The fans in all of our bathrooms our old and noisy.
I would like to replace them with a newer quieter model like the
Panasonics.

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?

Thanks