As long as your child still fits within the rearfacing limits of the Boulevard, it would be best to install it that way on the plane. The carseat may be a little more upright then you are used to since there isn't a whole lot of space on the plane, but it should be safe. I prefer rearfacing on the plane because my child is able to look at me so I can entertain her during the flight. If your child is still under 20 pounds, your child must still be rearfacing since your seat doesn't allow forward facing before that weight.
Print off these FAA documents on using a seat according the the manufacturer instructions and store it in your diaper bag. These documents may help you if you happen to get a flight attendant who insists that you forward face, even when your daughter still fits rearfacing.
http://www.airweb.faa.gov/Regulatory_and_Guidance_Library/rgAdvisoryCircular.nsf/0/60d70126cf679d5a8625723b007841e7/$FILE/AC%20120-87A.pdf
Prior to your flight, make sure that you bring along your carseat manual and locate the "FAA Approved for airplane use" sticker on the carseat.
If you were to move the child to forward facing, you would also probably have to change the straps since they would have to be at or above the shoulders, when you currently have it setup for at or below the shoudlers since your child is still rearfacing.
All the above information applies only if you bought a ticket for your child on the plane. If you are just hoping to get an extra seat on the plane and brining your seat along just in case, there is a chance that you will not be allowed to use your carseat as you want it to be installed.