We live in a mobile home (approx 27 yrs old); the bathroom off the hallway has an exhaust fan that is a 50 cfm (also original to home) that doesn't work well to get rid of the moisture/condensation when used for a shower.
I want to replace this fan with a larger one (& hopefully quieter one) a 100 cfm
Fan only unit (no light) which replaces the old one. Is there any reason why these two fans can not be switched (the 50cfm to 100cfm)?

The only problems would be the physical size of the fan, and the size of the duct.
You'd have to remove the fan to see what is above and around it to see if the new one will fit.
Also, 50cfm fans usually have a 3" duct, where a 100cfm will have 4".