The reason why FM is in stages is because the FM changes its nutritional components with respect to the age of the child. For eg, stage 1 FM usually has more fat because the baby needs it, whereas stage 2 FM will have less. Similarly, after 1 yo, protein is very important so stage 3 FM tends to contain more protein.
You can continue to give her stage 2 for a while. There's no hard and fast rule but it wouldn't be advisable to stick with stage 2 for too long. You can drag a month or two if you want to make sure your daughter adapts well.