You can switch to a convertible seat sooner if you want because many of them are safe for infants too if they’re rearfacing. But you don’t necessarily have to switch to a convertible seat until the infant seat size requirements are met. We switched to a convertible seat at 1 year just because our son no longer looked comfortable and we weren’t carrying him around in it anymore anyway.

Check your manual. Usually it goes by either the weight limit or if their head is an inch from the top of the seat.

Depends on the carrier. You could look it up online. I have the Graco 35. Max height is 32" and max weight is 35#. I typically move my kiddos up to the regular rear facing seat between 6-9 months. When it becomes harder to carry them in a carrier than to take them in and out of a regular car seat.