If you are using site-finished hardwood (both old and new), as long as the species and stain are the same, it should be fine. The boards are butted up against each other so close that you kind of lose the appearance of the width. If you're trying to match prefinished flooring, then no, it's not going to look good.

But it begs the question of why you want different width planks?

I guess I need context and rarely IMO do prefinished planks match up with solid flooring so at least show both choices and close match IMO usually screams not quite there . So along with a new plank size you have 2 different colored planks . I need pics of the choices and ideally a floor plan showing where the floors meet .I too wonder why the 2 different widths it seems to be making the whole thing more complicated than it needs to be . If the old floor is trully old the woods will probably not even look the same .

You don't say species, you don't have pics of the existing floor, nor the reason you want a board width change

I'll just say this:

Nothing unifies a home like consistent flooring. : )

There is no "kind of close". there is match and mismatch.

Thank you all for your feedback!

Additional information:

1. The foyer and dining room currently have 5 inch prefinished Brazilian cherry solid hardwood floors that were installed by the previous owner few years ago.

2. The new floors in the other rooms will be prefinished, as well. Site finishing is not an option.

3. Brazilian cherry has a lot of variation in color. Additionally, it darkens quite a bit from exposure to sun. Looking at samples is not very helpful.

4. There are two main reasons why I prefer planks narrower than 5 inches:

A) The living room and family room where the new floors will be installed are each about 250 square feet. The ceilings are 8 feet. The layout of the house is not open floor. IMO, narrower widths look better.

B) I live in northeast US where humidity levels fluctuate quite a bit between seasons. Going with narrower widths will help with seasonal expansions and contractions. The new floors will be solid hardwood. I have not been able to find suitable engineered Brazilian cherry floors

I understand that it would be better to rip out the existing BR flooring and install the same flooring in every room. However, I am not quite comfortable with removing 500 square feet of perfectly fine floors, so I am trying to see if I can find an acceptable solution. It will not be perfect, I understand, but OK is sufficient.

Forgot to mention, we will be repainting the walls, installing new baseboards and door/window casings, and the replacing the stairs/rails.