changing milk powder

hi mummies,my LO have been drinking stage 1 enfamil ,im a new mum, i didnt knw i need to change to stage 2 after 6mth... when i found out ,my baby is abt 10mth old so when i called the enfamil hotline , the person told me i can still feed her till she reached 1yr, she then ask me to change gradually to stage 3 and skip the stage 2, telling me not to mix together with stage 1and 3 milk powder, now my LO just reached 1yr old , she reject the stage 3 milk powder and she still prefer the stage 1 milk powder, so what shld i do now? continue to give the stage 1 milk powder? or change to stage 2 then slowly to stage 3? or try changing another formula brand of stage 3? #advicepls