Oh dear.
One thing a lot of people seem to agree on is that if you give chins the "mixed food" sort of food packs, they WILL become picky and only eat the delicious parts. I bought a pack of Versailles lage chinchilla nature, opened it, took a look inside, and got rid of the whole thing.
The worst part is, they will eat the ones that are bad for them, and leave the ones which are necessary for their health.
If it hasn't been TOO long, I'd try to go cold turkey and give them nothing but hay and the PROPER pellets. Animals have a working survival instinct, like small children. If they're not really hungry, they will be picky about their food, but once the hunger pangs kick in, they eat whatever you put in front of them. ... Not absolutely certain that works for all chinchillas, but very few living creatures are stupid enough to starve to death when perfectly good food is in front of them, and chins are naturally highly intelligent.
...they're also intelligent enough to sulk, and throw tantrums, and go on hunger protests, though. So be wary.