How difficult or expensive would it be to replace damaged sections of hardwood?

Question

I am moving into an old house that had carpet. I removed the carpet to find hardwood with pet stains. Sections of the wood are very dark. I don't believe that most of these stains can be sanded out. If I have the rest of the hardwood sanded, how difficult would it be to replace the damaged boards. The hardwood is red oak. Also, would it match or look like it has been patched?

Answer 1

Here's a DIY answer to your question:

http://www.diynetwork.com/diy/fc_hardwood/article/0,2037,DIY_13813_2269584,00.html

Answer 2

It is difficult and expensive to repair an existing floor. It depends on how many boards have to be removed and replaced with new. Wood is a natural product, so the replacement wood may not match in grain and color. Further, if the old floor had ever been sanded, the new boards may be thicker than the existing and might have to be milled.
Sometimes it is possible to salvage hardwood from an area not needed or not visible, like a nearby closet floor or a small side room. Patch the living room floor then replace the floor where the salvage was taken.
It would be way cheaper and easier to install new carpeting.