Car seat

When is a good time to switch my babies car seat from infant car seat to rear facing convertible? He’s too heavy so I leave the infant seat in the car anyway but it’s convenient sometimes to just click it into the stroller if He’s asleep or something. He still had 3 more inches to go until he grows out of it and 35 pounds is the weight limit he’s 20lbs). He’s also 9 months old. The convertible car seat just seems more padded and safer but I like the convenience of being able to carry him in the infant seat (but only on occasion) opines? Should I switch or should I wait?