Fun fact: Any original fir in that house is quite possibly old growth, which means it's denser and better quality than any fir you could get today (which is harvested from young, fast-growing trees). If the fir flooring is in good condition and looks good and you like it, I certainly wouldn't replace it, but it depends on how important the perfectly even floors are to you. (Make sure you use felt feet for your furniture and wheelie chair pads for any wheelie chairs - this will help immensely with the preservation of your fir floor. I know what you're dealing with because I have old pine in my attic.)

Is it for the look and the satisfying evenness of it all? In your shoes, I'd just get the nice threshold pieces to transition the floor levels more smoothly, but I'm not too bothered by having some thresholds. It's the nature of older houses.

Do you have someone in your household who uses a wheelchair or walker? In this case, definitely make the floors even, even if means sacrificing the perfectly good fir floor (which you should definitely sell/donate to salvage). Accessibility is important. But barring this scenario, I personally don't think it's worth it to replace flooring just to get everything at the same level. But I'm not you.

I'm dealing with this right now - some rooms upstairs in my house are missing the oak top floor and just have grody carpet. The existing hardwood is 5/16 inch thickness, and I want to get more 5/16' hardwood so the new and old floors are more or less even. My dad tells me to get 3/4 inch for the other rooms because it's a better option and to just deal with the height difference. Either way I have to do threshold divider things because I'm joining new floors with old. Ugh, decisions.

I don’t understand there being no sub-floor in the kitchen? Its a little hard to understand without pictures or a floor plan to know how the rooms, floor material and floor heights flow together.

I would want the floors level. It is a tripping hazzard to have them unlevel, unless it is designed as a step down or step up. I have a friend that had the step up to almost every room in her house, and many people tripped on those step ups and step downs.

Good point—thanks!

That’s what I was thinking about too; if the wood is still in pretty good shape after almost 100 years, it has to be pretty durable.

Removing that old floor would be a sin. Make a transition strip and forget it.

Yes, I’m thinking that too. Living a with a little transition is just going to be par for the course.

Richfield, I apologize for my unclear description. What I meant to write was that the old kitchen flooring (linoleum) was placed directly on top of the subfloor, which now makes the subfloor the same level as the rooms (dining and living) that it runs into.

Keep the fir. We have 100 year old fir floors (and our living and dining are oak as well, original to the house) and had them refinished a couple years ago, and they are one of my favorite things in our house.