I am glad you are wanting to keep your daughter rear facing for longer - we like to see age 2 at a minimum, and definitely longer if you can swing it.
There are seats that rear face longer for height. If the below doesn't help you, let us know what vehicle you drive, and what the other kids are in and their ages and weights (torso height helps too - measure from bum to shoulder while sitting).
The newer generation of Britax seats don't give an overall height requirement, but instead require 1" of hard shell above the head. I do not know if they applied this retroactively, or at all to the older generation (referred to here as 'classic marathon'). Some people would make the parental decision to ignore the overall height limit and go with the 1" rule, but I would suggest calling Britax and seeing what they have to say about it. I'm guessing a mid-July 2007 manufacture date? That would mean it expires mid-July 2013 - judging by her weight I'm thinking she probably has quite a lot of room above her head still, and if another year and a half of rear facing time is sufficient for you, you might not need another seat. But first, find out Britax's stance on it, and please let us know!