How quickly does the fat content in breast milk change? I have a frozen stash that I am cycling through that is about a month old. Is the nutrition in it going to be significantly different already?

I had been freezing 6oz at a time and spreading it over 4 bottles with 1.5oz of frozen and 1oz of fresh milk. I wonder if it is necessary to mix it so soon. LO sometimes only drinks 3 bottles a daycare, so I have to feed her the extra bottle that night if it is mixed. If I don't have to split it between 4 bottles, then I could just save it for the next day.

I'm just trying not to give her expressed milk if I could just nurse her and save the milk for her next day at daycare.