I agree with @Margrethe . For context, I used to make historic costume, so have a lot of experience sewing fancy frocks, including entirely by hand. It's next to impossible to alter the armholes and the neckline, because the fabric you need to make these smaller is no longer there.
Even nipping in the waist isn't that easy. On these dresses, the bottom of the bodice joins onto a waistband, then the skirt joins into that. To take that waistband in, you have to remove it from both bodice and skirt. But that leaves the bottom of the bodice and the top of the skirt too big to put back on the waistband.
The skirt is easier to fudge. It was probably pleated in anyway. For knife pleats or box pleats (the latter is what Catherine's bridesmaids dresses have) the top of the skirt needs to be three times the length of whatever you're fixing it to.
For gathers, less. For cartridge pleats more, sometimes significantly more. If you change the pleat style or overlap the pleats you'll change on how the skirt flares. But it's fudgeable. Having said that, one of the photos of Charlotte shows this was botched - the centre line of the pleats is off-centre on the dress itself, so it looks skewed.
The bodice is harder. It probably has side seams. If you take it in there, it impacts on the armhole size and shape. Which means you need to remove and then reset the sleeves. You can add extra darts elsewhere, but this changes the overall bodice design/shape, and may impact on neckline fit. You can gather the bottom of the bodice to the new smaller waistband. It looks like this last was done with Charlotte's dress but, again, not evenly.
Effectively, any significant alteration in size means you're taking the whole thing apart then rebuilding it. In a high shine, white fabric, which will show every mark. Where some of the fabric you need is just ... not there any more.
100% remake territory.