It's possible to change partners for chinchillas, but how long has your original pair been together? If they have been together for 3 yrs, I think you should leave them be. Some chinchilla pairs take years to produce kits, and it maybe an environmental condition causing it, actually. I've heard of pairs producing every year, and heard of pairs taking 2-3 yrs to produce 1 litter. It's not fixed.

What I mean by environmental condition is that chins may not breed if they don't feel secure in their surroundings. For eg, you may place their cage in a spot where there's lots of human traffic or alot of noise. The female may not feel secure and may not want to have kits.

Also, if your female is not in prime condition, she may not come into heat at all. It's nature's way of protecting the mother chin.

Chinchillas are bred in pairs and in polygamous groups in US. But those ppl who breed polygamously are experienced with pairing chins who do not know each other. So there is very little risk involved. You can give it a try if you want, but be fully prepared for the female to reject the male. You don't want fights to lead to injuries, in your bid to get your female pregnant.