"We installed hardwood over existing hardwood going in the same direction - although there was no subfloor, so pulling it up wasnt really an option. No problems at all." You had no subfloor? The hardwood was nailed every 16" directly to the joists? Did you do the work or did some contractor convince you that this was the situation? I Have not seen that in 40 years of remodeling old homes and will probably never see it. As I said before, TAKE UP THE OLD FLOOR!

It depends on what type of wood you have and how old it is. In our last house we had 75 year-old birch, and the new planks we added during a kitchen reno always looked a little different (although not enough to really matter...the planks had differentiations even amongst the original wood.) Of course we did refinish the entire floor at the same time... Oak might be easier! In our current house, there are 60 year old red oak floors. They'd been recently refinished (~3 years ago.) We needed to patch in a large section during our kitchen reno, as we removed walls and relocated the kitchen. We simply bought new unfinished red oak (at Home Depot) and a satin poly my DH hoped would match the existing. He expected to have to try a few different finishes and/or stain, but it actually looks identical just as-is. I can tell you----- staining is not the best answer for the long term!! In another part of this house, a previous renovator used WHITE oak, and then stained it to look more like the red oak everywhere else. When we tried to do a screen-and-recoat recently, when adding some new flooring to an adjacent room, we found we were sanding off the stain. So we had to proceed VERY carefully so as not to remove the stain.... otherwise it would have meant a major sanding of ALL the stain, and then re-staining. It was a bummer because there were some scratches and fading that I'd really wanted to fix, but doing so just would have been too big a project. If you want to be able to do easy screen-and-recoats, selectively sanding more where you need to for repairs, DON'T use stain.

I had the exact same issue. Initially they thought they could do it. But my house is old and when they did the demo they said the way they did the spacing of the supports under the wood on the older section cannot be duplicated today and the floor would never look even and the old section would always be a little higher and "squeaky". Ended up doing the whole floor. However, the hallways are old hardwood as well and the transition (via a doorway) seems fine.

Can you put down the birch and then put rugs over it to protect it from your family? That way buyers will see the hardwoods under the rugs but large portions of the wood at least would be protected. I agree with you about not spending too much on hardwoods but if the floor is as mismatched as you say you really should to change it. The home we are purchasing was built in 1972 and needs cosmetic updates EVERYWHERE including the flooring (one room has 3 different kinds of flooring - in this one room!). I like, am good at DIY projects and have the benefit of having family and friends in the construction/design business but the house was on the market way longer than others in the neighborhood and the final PP is $40k less than the house two doors down which sold earlier this year. Why? That house had been updated.