Changing color of existing wood floor?

Depends on the species of wood and the type of stain used. Oak, for example, is very porous; maple not.

It is possible to sand and refinsh, yes, if the floor has sufficient thickness--that is, if it hasn't been refinished previously too many times. You may be able to bleach it some (oxalic acid/wood bleach) but if it is porous probably won't be able to remove all traces. Best bet is to try an area such a closet and see what can accomplish.

Yes, it can be sanded out. But why stain it again, lighter color or no?

I've done it with great results using an orbital style floor sander.

It depends on why it is dark. Varnish, for example, often darkens with age and is easily removed. If it is a dark wood, and you can remove all finish, it can be bleached. If it is stained, that is more of a problem, as stain usually does not soak into the wood to a uniform depth, so you may sand, but only remove the stain on some areas of the floor, leaving a rather mottled look.

I would suggest you f> If you have a hardwood floor that has a relatively dark color, is it

If it is very old, it may be darker because it has old wax with dirt embedded. That can be cleaned off with mineral spirits. Oak doesn't finish to a light color, like the maple that is so popular nowadays. With plain, clear finish it will be a medium brown and tends to yellow a bit with sun exposure. For a real "light" color, a pickled finish might be needed. Stripping can remove most old stain, but still leaves a wood that is medium brown when finished. I've tried to bleach oak, with no noticeable effect.

Stain penetration depends on several factors. Most hardwood stain is rather shallow and it can be sanded out. It's a lot of work--a floor sander and belt sander will help. Close off the room (use plastic or whatever) else you will have dust all over the house.