There are a couple of way to tackle the high round back which either introduce a centre back seam, gathers across the centre back neck or a shoulder dart. Here's a really good Threads article showing the various options.
I tend to go for the shaped centre back seam and add about an inch at the deepest part of that wedge.
Threads don't seem to cover the opposite side of this which is that at the front I now have too much fabric between the shoulders and the bust, which fits ok out at the shoulders but is too much fabric in the centre. I remove this with a wedge taken out across the front between the bust and the shoulders. This works well on an open V or scoop neckline but on a round neck it needs to be redrawn lower down.
This alters all of the facing, neckband and collar pieces too which also need an amount removing where they would be attached at an area which has had fabric removed from the front or back. I often avoid things with a collar for this reason because you are then altering the collar, facings etc as well which is a pain.
Here's an example from a top I made in March 2014 from Vogue 2989.
I add a wedge across the back which adds an extra inch. And I also take a little bit off the top of the centre back seam. This is a super easy alteration on things which have a centre back seam.
Basically draw a line across the upper back (I do mine about half way up the armhole) at 90 degrees to the grainline. Cut through this from the centre back almost to the armhole leaving a small 'hinge'. Then add extra tissue behind and spread the upper section by the amount you need - 1" in my case. I also curve in the very top of the neckline (not clear on my photo) but same as the image below. That little curve affects the shape of any back neck facing and/or collar pieces, so you'll have to take matching wedges out of them.
Here's someone else's diagram with extra red lines. My alteration is more extreme and further down the back, but you get the idea.
This will then need a matching forward head alteration to remove the excess from the front neckline.
Again draw the line at 90 degrees to the grainline. Then mark 1" up from the line and draw a second line at an angle back to the hinge point in the armhole. Fold so the two lines meet and tape in place. Alter any facing and collar pieces to match (easiest done by transfering the two lines onto the facing piece and then folding afterwards).
This leaves you with a bit of a step in the edge and you can fill in with tissue if liked. I tend to just mentally add the tissue and cut accordingly.
This leaves the armholes and sleeves exactly the same and thats not been a problem for me.
I thought for ages I needed a small bust alteration but I don't - the excess fabric for me is between the bust and the neck so this is where I need to remove it.
I tend to pick styles with scoop or V neck as I don't like the sensation of a neckline at the collarbone level. However if you do have a higher round neck style and do this alteration, you will need to redraw the front neckline slightly lower, and alter any facing/collar pieces accordingly.
I hope this helps!
The top I made in these pictures was too drapey for the pattern and I didn't keep it, but I used the alterations for a dress as well, also from Vogue 2989, and here's an old photo of me wearing it.