Can I knit the pattern (Sweet Dreams 9896 -V-neck) up to the raglan, then stockinette for the top part of the front, back, and sleeves? Would doing this alter the size?
Could you please post a link to the pattern? There are lots of patterns called Sweet Dreams. More info is needed. What comes before the raglan lines?
I think it must be this one:
You could use a swatch (or the sweater body) to measure the pattern stitch gauge, then a separate swatch in stockinette to compare the gauge and this will help you decide if the size will change significantly and what you can do about it.
If the gauge is different (which it may well be with the stitch changing) you can make an inc/dec row at the transition between pattern stitch and stockinette and then you’d need to take the stitch count change into account for the raglan shaping as you’d be modifying the pattern.
Another option might be to alter the size of needle for the stockinette so that the gauge matches. This would mean the stitch count remained the same and you could follow the raglan decreases as given in the pattern, but you would need to decide if the different gauge stockinette produced a fabric you liked (not too open or too tight).
Either way you’d need to do some checking for the modification, but you can do it.
I rather like stockinette on a larger needle which gives a softer fabric but not everyone would like it, the stitches are larger. I’m not keen on a smaller needle as it makes the fabric stiffer. But it’s personal preference.