We are installing a new 6 sliding glass door to replace an existing window.
Header above sliding glass door.
So for piece of mind i am leaning toward replacing the header to avoid future problems.
The header is usually made out of dimensional lumber installed on its edge.
Also be aware double or triple jack studs may be required based on the opening width and the load the header carries.
A 36 door has a rough opening width of 38 so your header is cut at 41.
The existing header is 2 2x8 and is actually long enough for the new door.
Calculating window and door header requirements is complicated because they depend on a variety of factors including the size of the opening the weight of everything above it snow loads if it s an interior or exterior wall and more.
However all the header sizing tables i have seen require 2 2x10 for this size opening.
If you have a floor above you ll need 2 2x6 s unless the building is 36 wide in which case you ll need 2 2x8 s.
Doors in a load bearing wall that holds the weight of the house created by beams and trusses need a larger header than those in non load bearing walls.
For header spans see tables r502 5 1 and r502 5 2.
Window and door header requirements.
The header for a door is much like a bridge transferring the weight above it to the floor and foundation below.
A typical header width with single jack studs is cut 3 larger than the rough opening.