Walls and Ceilings - door header modification in plaster wall

We have a 1930s cape cod, and there is a doorway to the attic that is 6'0" (Where the others are normal height, and the ceilings are 9'0). The angle this creates does not allow us to move a queen mattress upstairs, and the stairwell is kind of dark. The walls are plaster on wood lathe. I'm wondering if it would be difficult to cut away a portion of the plaster and move the header up, to match the windows, maybe?
Thanks,
D

Of course. I think the header height is 82" for a pre hung door.

ok ;) stupid question. Of course it will be hard. What I would like to know are methods for cutting through the plaster on wood lath, without damaging the surrounding area, and reframing the door to be at least a foot or so higher. I knocked around a bit and the piece of the wall over the stairwell door sounds hollow, as if there are no studs in it, all the way up to the ceiling. Could maybe install a landing, then door to the window height and a transom to the ceiling?

I find it easier to make the cut and not worry about the plaster then restore the plaster. But that's just me. I think the least destructive way is to use a masonry blade to cut the plaster then a very fine tooth circular saw to cut the lath. It will be unbelievably dirty. Wear a respirator. A less messy but more risky way is to use fine teeth in a reciprocating saw. You have figured out that vibration of the lath will break the keys beyond the cut and loosen tha plaster. Finer teeth will cut more and vibrate less. Get a good supply of blades. Plaster is very abrasive to blades. Though I've never tried this it might work. Cut the plaster with the masonry blade. Take the plaster off the lath on the inside of the cut. Then use an anvil pruner to cut the lath.

Also, consider getting the mattress in before you frame the door back in. The opening will be just that much larger. Cover every thing and seal all the openings you can before cutting. You will not even be able to see for an hour or so after the cut until the dust settles. WEAR A RESPIRATOR. CLEAR THE HOUSE OF OCCUPANTS TRY TO SEAL THE WORK AREA FROM THE REST OF THE HOUSE. Anything you do will help but nothing you do will be enough.

thanks, good to know :)

Alright, I went and did it, and it was easier than I expected.
One thing that made it easier was that one side of the wall was drywall.
1. I ripped off the drywall where I wanted to raise the door header. This gave me access to the back of the plaster, where I could see the wood lath.
2. I then used a drill to make holes in the plaster ( not through the wood lath) at the edges of the cutout and at the stud in the center.
3. I chipped off any plaster that was hanging through the lath.
4. On the room side of the wall, I then score-cut the plaster with a jab saw from hole to hole , and was able to remove it with out disturbing the wood lath to much.
5. I used a harp saw to cut out the lath, and a hack saw to trim the stud, and ended up with a very clean opening.

Of course this would probably be faster with a power saw (but sloppier?) of some sort, but the tools were very inexpensive.

Is there anything I should do to the edge of the cut plaster? It will be tucked under ~4" of trim.

Just case it out.
I suppose you could squirt some glue between any lath and plaster that have separated but about 4" is a lot of protection.
Thanks for checking back in to let us know how it went. Not too many people follow up on their progress and results.

Yes. I hope this might be helpful to someone else. Thanks, for the help.
I built out the bottom step of the stairs as a landing and raised the door jamb onto it. The next step on this project will be finding or making matching trim (very simple but very old stuff) to extend the door jamb up a foot.