The Disguise Self spell states:
You make yourself - including your clothing, armor, weapons, and other belongings on your person - look different until the spell ends or until you use your action to dismiss it.
A cursory glance at the description has always left me with the impression that you cast it, choose your new appearance, and then you are stuck with that appearance until the spell ends, you cast another, or you dismiss it.
However, one does not have to read it that way. The spell indicates you can make yourself look different until the spell ends. It doesn't say you can't constantly change your appearance during that time.
So for example, could I cast the spell to look like a servant, then change my mind and look like a guard?