If there is some reason you would need another Marathon anyway, I'd do it. Like if you are planning on getting a seat for another car or have younger twins they could both be passed onto or something.
Otherwise, I'd feel ok turning him. My daughter was about 2 years 9 months when she had to be turned for the same reason. Mine also hit a growth spurt and gained those last 2 lbs very quickly.
I'd also just turn the Marathon FF instead of getting another seat right now. It's very easy to use FF - just remember to switch the direction of the LATCH connectors if you use LATCH.
btw: I'm not a tech, just opinionated...