I Have a small bathroom and it has a light that is separate from the exhaust fan. I want to upgrade it to a light and fan combo. Can this be done easily without having to go into the attic? I currently have the original exhaust fan from when the house was built in 1966. I already have three light switches in the bathroom.
Old school: probably not, unless the guy who wired that ceiling was generous with the wire.
Cool new tech: Yes. They make electronic switch controls which are designed to solve a different problem: "I want a light AND fan, but my bathroom is wired for a light only." This consists of a smart switch that goes in the site of the regular switch, and an electronic control module that sits in the fan housing.
Now, I'm guessing your old fan has a vent - that is to say, a pipe leading from the top/back of the fan to the great outdoors. That will be a consideration. Which is to say, if your fan/light combo is going where the fan was, that's not a worry. But otherwise, you will need to be up in the attic reconfiguring that pipe... and while you're up there, you might as well rearrange the wiring to your preference also.
If you want to control the new fan and light separately you will have to do some re-wiring, and this may be difficult depending on where the existing light is. If you don't mind them coming on together then the wiring part is quite easy.
Whether you have to go in the attic depends on the fan you bought. Most do not install from below without hassle. Some, Panasonic in particular, do install from below quite easily. The venting is another issue, be sure to reconnect the vent and make sure it is not obstructed and vents to the outside.