I COMPLETELY agree with the previous poster that you should leave your daughter in a rear facing seat. This is from the babies r us website:
"The American Academy of Pediatrics recommends that babies ride rear-facing until they reach at least one year of age and 20 lbs., but ideally longer if possible within the rear-facing limits of their seat. When your child exceeds the infant car seat's height or weight limits, switch to a convertible car seat. This will allow your child to continue to ride rear-facing up to a higher height or weight limit before having to switch to forward-facing car seats."
So, this may explain why your pediatrician said to switch seats? Is your daughter too tall for your current seat? If the answer to that is no, then I say ignore your pediatrician, because s/he is not up on the current guidelines. Since they change all the time, that is not entirely surprising.
The bottom line is that you should leave them rear-facing for as long as possible. My eldest son was 14 months when we switched him, because that is when he reached the weight limit for his seat (25 lbs). So, the AAP (and every other child organization) says that the guidelines are one year of age AND at least 20 lbs. If you have to get a new seat because your daughter is too tall for her current seat then get a convertible seat, as Babies R' US suggests. That way, you can leave your daughter rear facing until she reaches the weight requirement and only have to buy one new seat.
Please go look at this video on you tube, showing the difference in crash tests in rear and front facing seats. A mom made this for her sister to convince her of the very same thing that we're discussing here:
Also, you might share it with your pediatrician...
Hope this helps!