Yes, you can switch him to a rear facing convertible but at 10 pounds some parents find they can't get as good of a fit and the seat has to be at the maximum recline. You are allowed to use rolled towels to help support the baby but do not attach any other after market devices sold...they are not safe...and try to keep him rear facing until the limits of the seat (usually about 35 pounds check the seat). Just curious, what infant seat do you have???? I loved the convenience of mine with both kids...I never carried it though, always used it with my Combi stroller.
The Fisher Price Safe Voyage is made by Britax and is a good seat...BUT they stopped making them about a year ago..so any seat you buy will already have one less year of use and if you need a part it will be almost impossible to get one...other good choices are any of the Britax's Marathon, Boulevard, Decathalon. One of those has an infant insert I believe...that may give you a better fit for your little one. (bummer you just missed their huge sale they were dirt cheap), but still they can be had for about 229 online, other recommended seats are the Evenflo Triumph ADVANCE, it harnesses to 50 pounds and has good review. My personal choice- the Marathon, I have 4 of them and have no complaints, Britax seats are so much easier to use,extra's that make life much easier, comfy for kids and have lots of cover patterns (hey it's nice to choose something cute as you'll be looking at it everyday for a long time!).
Good luck with your search!