You don't need to change it unless you want to. I used the same pram from birth to age 3 with my first daughter. This was the one that came with the original travel system. Re car seat, most infant carriers last til over 1 year, but depends on what one you have and how big/small your baby is. Just need to adjust the headrest accordingly and remove inserts. It's normally safest to keep them in these as long as possible.
We’ve just changed the car seat as she was getting too big at 7 months. We got joie spin 360 and it’s great
I got the mybabie one and love it. Light weight, reclines, folds down small
I also need a new pram. If you’re from London the baby show is on this Friday and then have a wide range of prams on display for you to try out. They usually have some decent discounts on the prams