A bifold door, even when fully open will take up about 2 of the total opening, depending on the panels...could be even more. A swing type door will do the same if it has to open against a wall at 90 degrees to the opening. If the swing door can open fully, then it should not impact the opening at all, although it requires door stop, which is about 3/8 (x 2, need it all around the door).

I think we have a terminology problem. smoenra...instead of casements..do you mean casing? The trim that is flush to the interior wall around the window? And jambs are normally an integral part of a window, though they could have been added after install. Could we be talking about sills? Or more correctly stool? Also, common usage of replacement window is as Bill said. It's an insert that is normally used to replace the moving glass sashes in a wood or metal window. If you are prepared to remove exterior trim and siding, then no reason (other than shimming) why new construction vinyl windows couldn't be used. This is assuming the jamb is not attached to the existing window. If exterior is peeled back, then flashing, caulk and tape can easily be applied. And you said you have vinyl windows? Are they vinyl clad wood windows? If not, why do they need to be replaced?

Yes, we do have a terminology problem, sorry. My house is only 9-yrs old. The windows were made by Care-Free, which is out of business and most of the windows are either very drafty or have glass seals that are broken as we have mold growing on several. The wood that I wish not to remove is the casing and sills/stool. From the inside of my house I can just barely see 1/8 of the window frame. My plan was to keep all of the wood inside the house un-touched and remove the vinyl (not wood clad vinyl) windows from the outside of the house, yes, I plan on removing j-channel and siding to do so. My question is if I use a replacement window (w/o nail fin) how do I seal the area between the window and underneith the j-channel? thanks for all the replies!