I would definitely switch now. I think I switched at 4 months or so with my bengals, and Neytiri lasted all of about 2 weeks on kitten food before graduating to adult food (mostly out of necessity because she wanted to eat with the bengals). It doesn't sound like necessarily an excessive amount of food that you are feeding them, although switching to an adult formula and maybe reducing the dry food a bit might help.
I'm going through something similar with Gaga right now. The vet asked me to slim her down a bit. It's tough because I've been out of town much of the last 2 months, so I think all three have over-eaten during that time (and Gaga is definitely the most susceptible to that), so now we're getting back to a more normal routine and I'm cutting the dry food and snacks down quite a bit, and trying to ramp the exercise up.
This is a good read:
http://www.littlebigcat.com/health/feline-obesity/