A step by step process of some boring work that no one besides me will notice the difference!
When I first made this dress, the back V went down way too low past my waist. Gave it a much more 18th-century vibe than I was looking for. So, since I needed to take the skirt off anyways to add more fabric, I figured this was a good time to fix that as well.
First, I unpicked the bias facing.
Next, drew a line to make the new back point:

Step not shown – unpicking the seam for the bone, trimming down the bone, and resewing the channel. Times like these I’m glad I used cable ties which are super easy to cut.

Here are the bits left after cutting. The blue and green are both cotton canvas remnants from Joann, and the pink is an annoying-loose weave linen-look that I picked up for a dollar at a theater sale.
It’s not just the camera angle which make those triangles look wildly different. Somehow they ended up totally different shapes. When holding the bodice up to the form, the sides didn’t look quite even, so I freehand(!) trimmed it with scissors until it seemed ok.
For some reason I had in my head that I was going to have to cut and sew a new bias facing. But then I got hit with the clue-bat, and just folded my main fabric over the lining and whip stitched it down.
And really this only ended up taking ~1 hour or so. Really much faster than I expected. Hooray!