the attached article gives some data;

Storage of human milk and the influence of procedures on

immunological components of human milk

RA  Lawrence,Acta Pñdiatr 88: 14-8. 1999

if the storage conditions are very stable, you may not find a significant change.

It also depends on container (materials, surface to volume ratio) especially for fat content. Please see some hint from attached reference (p.11).

Article Rapid measurement of macronutrients in breast milk: How reli...

Freezing and thawing might cause a big problem if you want to feed the milk to a baby, because it can give a terrible off-taste and smell. Even a few days storage in the freezer (home situation, -20degC) can be disastrous. Not all mothers milks appear affected by this (maybe it is linked to different lipase activity in the milk and/or different freezing/thawing conditions). So every mother should check this for herself if freezing fresh milk at her home situation is an option. I am not sure what gives the off-taste and smell, lipolysis after thawing, and increased oxidation of the PUFA could maybe also be part of it. If anyone has more information about this I would be interested know. Microbial hygiene does not seem to be a factor, since milk frozen for 2 day gave an off taste and smell, while the same milk stored in the refrigerator for 2 days was fine.

Storage and freezing will in a variable way affect composition esp.vitamines and peroxidation of lipids and proteins. Interaction with air, light and ssurfaces will Always have a minor effect. But do not forget what is done during production of formula or fortifiers. Most of human specific components are completely lacking and substituted by cow (ruminant with four stomachs) Components. So if fresh human milk is not available prefer freeced and stored mothers (or donated) human milk.