It depends on your seat, the weight requirements are different for all of them. If he's already 25 lb he's probably outgrown it unless it was brand new for this baby, some of the newer seats go higher for the infant carrier. If you told me the year and model I could find out.
Also, he must have at least 1" of plastic shell above his head when in the seat, even if he is still under the height or weight limit in the manual. Once he has less than 1" over his head, he's outgrown the seat.
You can switch him to RF in a convertible any time, he must be getting heavy to lug!
A good but not outrageous convertible would be a True Fit by The First Years ($150 at BRU right now), or a Graco My Ride 65 - a bit more ($230) but a nice seat. Evenflo Triumph is $160 at TRU.
For your daughter an Evenflo Chase will do the trick for not too much money. I am getting one for my 4 yr old who's 40 lbs for the grandparents' car. If you'd like suggestions for seats that RF to 40 lbs, your DD can still ride RF as well, let me know if you'd like more info.