Are bath fans standard sized and "upgradeable"?

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

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