We are now at the paper pattern and no- you need no new paper pattern and no CB seam, CB is on fold and stays on fold - but how should I namel it??
Lay your back pattern on a piece of paper. Fasten it with tape left and right at busthole line. Draw a guide line vertical up at CB, Then you can better move the pattern pieces.
You have at armhole a side dart. In your case where the yoke will be. For betterpattern manipulation you rotate this dart up into the shoulder.
How to do it: You cut the upper leg of the dart to dart point und then straight up to shoulder. To close the old dart you rotate this piece to the other dart leg. Rotating point is the dart point. A shoulder dart will open. Fix the rotated piece with tape (clear glueing tape). Now you cut horizontal across. You can do it hrough the dart point like it was a yoke. Because the dart is open, you get 2 pieces. (Normaly the armhole dart is in center of armhole height and later moved up for the yoke, but it does not matter so much where you cut). Mark 5 mm at the CB guide line up. Move both parts up together. Rotate now both down (rotation point is at dart point) so that the CB seam is closed and the armhole seam too. You have get an upside down V-shaped inset.
Now you see that the center piece's CB seam is angled. You don't want this.
You connect your guide line with the CB seam and you measure how much you have to take away. In our case it is 2.7 mm. You do a horizontal guide line from the neck shoulder Point. You mark on the guide line 0.27 cm. You redraw the neckhole form CB to New neck shoulder point. You measure the old shoulderline fron Neck point shoulder to the dart. You draw the new shoulder that is now the same length. This will shorten the dart intake.
This is the alteration for shoulder blades. I have left out the general going up at center as your pattern is sitting already high at the CB neck.
Ther vertical cut shows how the make your Backwidth smaller - like you did with the pinning out without to have draw the pattern new. You cut in under the armhole und up the shoulder . You rotate it in about the pinned amount - which should be 0.75 cm (right?) at hAe point. Fix it with tape. Then go down the same amount inward to waist point and to hem.
After this you can redraw your fabric back. You could use your old back nothing important has changed besides the shoulder slope which you should draw into the pattern. Just mark your new seam lines on and sew it new to the front part. But first check if your space under the pits is enough after the altering for more slope. A shirt can have 1,5 - 2 cm. Use your finers to check.
hell what a bunch of words