It depends on whether your windows are a stock size. If they are, and you choose just to replace the sashes, then that's all that changes. (But, even then you are likely to alter the muntin pattern, which does change the look of the window.)

However, many people change out the entire windows, including framing and trim, and unless you have a restoration carpenter working on this, the vast majority of windows look permanently altered.

And not, as you might gather from the chatter here, for the better!

The reason for the more major work is often economic as the replacement units come pre-sized and the existing opening doesn't fit with the new one.

Yes, of course, you could have custom-made exact reproductions made but they are very expensive. So when most people talk about replacement windows they are talking (railing against) about the few-sizes fit all, and none too well, commonly available ones.

In many, if not most, cases substantial energy efficiency gains and repairs of problems can be done without changing out your windows.

I've attached a link to a Preservation Brief on Energy Conservation in older buildings. There is an excellent one on window repair in the same free (you already paid for it with your taxes!) series from the National Park Service.

Molly~

Here is a link that might be useful: Preservation Brief on Energy Conservation in Older buildings

There are constant arguments daily on what is a replacement window and what isn't. My theory is anytime you change your window it's being replaced. Now there are three types of replacements we do on a daily basis. The first is the sash replacement kit like Marvins Tilt-Pac. These are just sash and jam liners that insert inside the old frame. The second is an insert which fits back inside the existing window frame. This is the most common practice used today. The inside and outside trim boards are left in place. Usually we wrap (or Cap) the outside with aluminum coil stock to make them maintenance free. The third is a Total Frame Replacement. This is by far the hardest of all for us to do. It takes a lot of time and correct measurements to make this all come out right. Most manufacturers will make a window to your specifications at a small mark up from stock sized windows. Marvin and Pella both have product made in eighth inch increments that usually have the size required out of stock choices. You just need a very skilled Pro to measure the existing windows for the correct sizing of the new. We use the Pella product more here because they have some great snap on accessories to keep the outside looking sharp and clean. Good Luck!!!

Thanks for the info about the windows!