is it too early to upgrade my baby's car seat to a convertible seat at 6 months old? she's currently 7kg (4 months old) and I will be passing on my maxi-cosi infant car seat to a friend in 2 months time. which seats will you recommend?

Babies grow pretty fast so upgrading the car seats out of safety is a good idea. I'd recommend brands like Graco or Britax. They are known for their safety standards, sturdiness and good workmanship. The Marathon car seat series from Britax are easily convertible - from front facing to rear facing.