Formula dilemma

so my daughter has a wic apt in the morning....sad thing is i only have a little bit of formula left (the one that we actually use) i do have a sample of Similac that we got but i use enfamil.... now idk if i will get enfamil or Similac on wic but my question is.....if i run out of the enfamil and i use the similac sample...do i need to use it all before switching back to enfamil???