not to ruin your day but dark wood... has new roofs and beams and decorative items. Your walls, floors, doors, and most everything will not change. There is a new door but its ginormous and won't replace normal interior viking tall doors, it replaces the 'gate'.
so apart from re-roofing, 95% of your house is most likely to remain untouched (though the % will be higher if you have an epic roof on a 1 story home)

moving from stone to marble is another story. Not only do you have to replace each block, but the blocks are not 1-to-1 replacements in most cases. This is a heroic effort, and basically either do-over or stone by stone it slowly.

here, the roof, wood poles, and arch pieces on the door are dark wood. The door is the standard 'gate', the walls are standard wood, etc. I think the bird decoration is dark wood, while the dragon is normal, and so on.
https://steamcommunity.com/sharedfiles/filedetails/?id=2924987564

Originally posted by jonnin:

moving from stone to marble is another story. Not only do you have to replace each block, but the blocks are not 1-to-1 replacements in most cases. This is a heroic effort, and basically either do-over or stone by stone it slowly.

I'm just slapping some marble on top or in front of stone. Nothing wrong with the good old-fashioned rock walls, and marble only adds some depth feature to them. Same thing with iron cages.

https://steamcommunity.com/sharedfiles/filedetails/?id=2931228522

Upgrading base buiilding parts is not possible in Valheim. You need to rebuild the whole house.

Well, while you can't actually upgrade building parts, you can just rebuild the parts, that you want to change, no need to tear it all down.

It's also possible to get a bit more creative. You can clip items inside others, this way you can for example have your basic wooden wall which hides an iron cage completely inside of it. It requires rebuilding the original piece, but I would consider it an upgrade of sorts even with all the extra clicking.

You won't ever lose materials used for building unless there's some sort of fuel involved (like in case braziers), so you can rebuild something as many times as you like.