Planing is not an option, they couldn't really be planed insitu and no one would pass old floor boards through a planer/ thicknesser anyway for fear of old nails, grit and other hard particles that would soon make a mess of a set of knives! You can have the boards sanded, lightly to remove surface discolouration and minor scratches etc. but this will affect the appearance; however the boards will still look "old" and in keeping with the house. You could hire someone to do this or try it yourself; you would need an industrial floor sander and edge sander (all available on hire) but this is dusty, tiring work. This has been covered in many other similar threads.