What milk to use when going back to work

I had started to pump the first week after my daughter was born, not very often (maybe once a day-a few times a week) in order to get used to the pump and also to have some stored milk just in case I needed to run out or got stuck at a doctor appointment, my husband had back up food for our girl. I know your supposed to defrost breastmilk using oldest first...but the early milk is darker/thicker and I know your milk changes as their dietary needs change...should I still use the Milk I pumped from the first two weeks of her life or ditch that and only use the more recently milk? She's currently 7 weeks old and I go back to work in 2 weeks. Thanks :)