I took a lovely RTW dress which had a very shallow boat neckline which was uncomfortable on me and altered it to be a modest V, still high enough to showcase my favourite bold necklaces, but now not uncomfortable across my throat.
steps for this alteration:-
Put the dress on and roughly mark the new desired neckline, then unpick the stay stitching on the lining, turn inside out and press the neckline seam flat.(I must say that even in good light unpicking the stay stitching was very difficult and took me ages).
Next measure the markings you made earlier and adjust if required, add tailors tacks.
When trying on garments in the changing room, try sitting down in, especially in trousers because you may find your weight shifts a bit. Also pop out of your little cubicle and stride up and down a bit. Then pop back in and reach for something on a pretend high shelf. If the garment still works in these situations as well as when standing looking in the mirror, you are good!