Are you using the same exact wood?

I don't know if I will be able to find it. It was down when we bought house. That part of my dilemma.

What you would have to do is find your new wood and replace the T moulding. The t moulding is the solid piece the is the transition piece between your current wood flooring and current carpet. So you new t moulding piece would be your transition between the two different colored woods.

If you really want it to look good, consider lacing in pieces across the doorway and then refinishing everything to match. Another option is to run a perimeter border around the den and then fill in the center with a close color match to the existing wood.

I had the same problem. Even though I knew the product that was down in my foyer and dining room when I bought the house, I could not figure out how to blend it successfully. Professional opinions were divided on the idea. I gave up & decided to keep carpet since it would have been an extremely expensive "oops" if it didn't work well.

You could match the wood type and refinish them to match or you could simply go with a different type and stain and put a transition strip between the two. Most reputable floor refinished can match and finish what you already have in your home

Thanks for all the answers.