Re: Cannot remove double hung sashes without demolition - Is this true?
The general process for removing old DH sash is as follows:
remove the inside stop bead, then the lower sash will come right out.
Free the upper sash because it has to be in the down position for the next step. This is where a lot of specialized tools can be handy, like a heat gun to help soften the paint all around the upper sash, and in its race beneath. Then a "window zipper" can be run around the perimeter both inside and outside, which breaks the paint bond in the races.
After the sash is lowered, the spline, as you call it (it is the parting bead) can be removed. If you destroy it in the process, you'll need to find or make a replacement; not an insurmountable problem.
On almost any house, removal will require some ladder work, which is why people tend to put off window work until it's too late.
Working windows are a real plus in an old house.
Remove not the ancient landmark, which your fathers have set.