It's not truly decentralized, it's federated. So if lemmy devs change things, each instance can choose whether to pull those in or continue with its current version, potentially defederating as necessary.
I mean instances are not decentralized, they are federated. When I say "decentralized," I mean how BitTorrent is decentralized, as in there's no central server where everything happens. Lemmy is federated, which means there are multiple centralized instances that communicate with each other.