You can get replacement windows that slide into the old opening. Then the interior casing is not touched.

Check the link below. It is one I found from Pella that shows how replacement windows are done on older double-hung windows.

Most replacement window companies do it like this. You do not need to purchase new construction windows which would require the tearing out of the whole frame. As long as your old window units are in good shape i would go this route.

Plus you do not need to purchase Pella windows to get replacements like this. Shop around the window suppliers in your area. Get what you want

Here is a link that might be useful: pella link

Thanks for the link. I will look into the Pella windows, as I really like them anyway. My problem is that my window openings are currently 150 years old. Some of the wooden windows have NO interior casing, stools, or aprons at all. The old wood windows and alumium storm windows are attached to the window frame and have a 3" windowsill on the OUTSIDE of the house, but inside, most of my windows have about a 15" plaster or drywall stool (or windowsill) and about the same 15" top, left, and right side "wall" that extends from the inside drywall wall to the window itself. Does this make sense? Most replacement windows I see are very much thinner than this, ie: only having about a 2-3" windowsill or stool. I want the replacement windows to sit "out" from my walls, and keep the 15" interior left, right, top, and bottom "sills" that big. Am I making much sense?

If you go with new construction windows in the openings,their are mfg's that recreate the look of the old style windows on the exterior. Check out http://www.oldhousejournal.com/ and This old House website for ideas.

On the interior extension jambs and stools can be made in the 15" width you need by a good trim carpenter with experience working on older houses.

Everything depends on what kind of shape your old windows are in,do you go with all new or just replacements.

First thing to do is find a contractor you trust to give you a honest opinion on the shape of your windows and what you should do. I doubt any of this is going to be cheap whichever way you go.

We do a couple installations on homes that are in that age category every year. Some of the homes we've are historical and must fall under certain guidelines & appearance when improvements are needed. Since your already fond of Pella then your right on track. Personally I find the Pella Architect or Designer Series window works the best in these applications. These windows are aluminum clad on the outside and wood on the inside. So you keep the wood on the inside, as your existing windows are and upgrade the exterior side to maintenance free aluminum. Pella carries a frame expander system that locks into the accessory groove on the outside of the window. This allows us to span and fill the homes exterior casing pockets with an aluminum covering. If the exterior brickmould (Casing) is over 8" then we bend aluminum to cover the old brickmould and lock it into the new window. The inside can be done by a skilled window installer, if available or a trim carpenter. Most window installation companies have a qualified installer who can meet your requirements. You'll just have to be patient and find one. Good Luck!!!