If you have the LED lamps, then yes, the whole assembly has to be replaced. They did make a "standard" version of the same lamps and replacing both sides with TYC or other aftermarket halogens is a lower-cost option, but your IPDM and possibly BCM would have to be reconfigured by a dealer after installation to prevent "headlight system" warnings. That can also be "dummy wired" on most Nissans by adding a relay, I can get you details if you need to go that route.
Moisture intrusion is an industry-wide issue with LED headlights, not just Nissan. While LED's produce great light with a fraction of the power used by halogens, they also don't produce enough heat to chase water vapor out of the headlight housing. Nissan and other OEM's suggest turning the high-beams on for a few hours when moisture inside a lens is noticeable, but many people don't use high beams much and simply don't notice till it's too late. Sealing the housings like a taillamp is not a practical option, because the other lamps are still conventional and expansion from heated air could split the housing open. To date there's no good engineering answer for that.